1) They're not saying they don't want to help investigate the SB shooters, only that the order illegally expands the use of the All Writs Act and sets a bad precedent for democracy.
2) If they beat the order then the FBI needs to find a new way to compel Apple to help them do shit. That likely means the FBI needs federal legislation passed, which in the current climate will buy Apple considerable time. This is why they want to beat the order (though the feds could further appeal all the way to the.... wait for it.... 8 judge supreme court!) Though I think Scalia would have be on our side on this one). It's not about this particular case since there will be others, it's not about this particular order since there will eventually be legislation.
3) So why go ahead and do it anyways? Naive (but still valid) reason: they happen to be able to help and without it being ordered they can do it without handing over a tool for ad hoc decryption. They can even go to Hawaii after and throw the dev machines into a volcano to satisfy their inner hobbit (I highly recommend this part of the plan) to ensure that no one can abuse the power of the one ring.
The non-naive answer is that for a quick project they get to show to the public that no, we the nerds are not so obsessed with abstract systems level thinking that we won't help when we can. It gets a lot harder for that moron Comey to hit the morning shows and throw shitty innuendo at the tech industry implying that we're aiding the terrorists.
Both encryption and terrorism are complicated subjects that are scary to the average American and although they distrust the government, they also distrust Silicon Valley. The cryptowars aren't about being right or they'd have stayed dead in the 90s where they belong. Basically Apple makes tech look like the good guy fighting terrorism, and for anyone who cares (smaller audience than the fighting terrorism bit) they also defended your civil liberties.
4) This trick only works on older devices. They will die out soon anyways. Newer devices are safe anyways. If they beat the order and do this one case voluntarily then no precedent is set, so they can't be bullied into doing it and old devices are safe.
One device compromised, all other devices safe, order beat, PR win, Comey looks like a prick even to the uninformed next time he insinuates that we're the enemy.
But this all hinges on the naive assumptions that this is a one off and will never happen again, and that later generation devices are immune from any circumvention attempt. History says this isn't how this plays out.
If you crack the encryption once you'll get orders to crack it again and again, and in much lower profile and lower stake cases. Look at the prevalence of espionage tactics such as Stingrays and "parallel construction" by law enforcement. There may not always be someone you can pump up into an crack international terrorist, but there's always some low level drug courier, or a "quality of life" criminal to use your new toys on.
Also you're saying hat this doesn't set a precedent, but it does. Sure there's not a court case to point to, but it's a precedent none the less. It's that the company not only has the means, but the will to do it. What's stopping the government from coming back a second time, or a third time about this? What argument do you have on either a legal court or the court of public opinion to make when you stand up and say, "That first time was an exigent situation, and so was the second, and the third... But this time, the fourteenth time, THIS TIME we really mean no more!"
Finally, I don't think this trick only works on older devices. The FBI wants them to be able to brute force the passcode through a USB connection instead of making some sort of robot to tap the screen a bunch of times. Also presumably the FBI wants the the two many incorrect attempts lockout feature disabled as well, otherwise their just going to be waiting for hours on end. Why wouldn't this rather low sophistication approach work? from a technical stand point this is no more complicated than a mouse jiggler[0]. Of you're arguing that iPhone 6=< have some sort of "Mission: Impossible" self destruct mechanism, I'm sure it could be disabled given enough resources and motivation.
Finally (for real this time!), making a big stink and then capitulating is never a PR win. You just look like a tool to everyone involved. To the anti-encryption side you're a weak and can be rolled, and to the pro-encryption side you're a sell out.
Both Paris and SB were great examples of the terrorists basically not bothering with using encryption, if they had actually used and benefited from encryption we'd be discussing how to get cryptography re-legalized.
I doubt there's even anything on the phone the FBI don't have from other sources. The reason they're using the All Writs Act with this case is because of the publicity of the case so they can point to Apple prioritizing some vague principle most people don't care about over real dead people. Apple's doing a good job making their argument to those who care about said vague principle, but not to the general public.
I think you're underestimating how badly we're going to be taken to the woodshed on this the first time it's opportune. My be it it'll be some cute little girl dies in a Nancy Grace friendly way and the FBI manages to convince the public that "if only we could have broken these messages" etc. It might even be true in that one freak instance, but then we'll have "[cute_little_girl.name]'s Law" which will make sure that such a tragedy never gets exploi.... reported again, by making sure that the government can read messages when they need to. The US market is too large not to capitulate at that point. That bill passes if the voting public can be stirred up against the greedy and aloof tech sector. It doesn't pass if they see the tech sector and its goals as reasonable, and while we should continue trying to educate people on why encryption is good and important for them, you don't change the number of minds we need to change with rational arguments (or again, we'd have won already).
Edit: very interesting link on the mouse jiggler though, thanks for that!
wrt to "Of you're arguing that iPhone 6=< have some sort of "Mission: Impossible" self destruct mechanism, I'm sure it could be disabled given enough resources and motivation."
The iPhone 5S and newer has a coprocessor (or co-computer) that has a hardware enforced rate limiter as part of one of the features of the "Secure Enclave" (which, word on the street is, cannot be overridden by software).
EDIT:
It's not just physical access. Physical access chains the game entirely, but what the FBI is wanting highlights the physical access problem even more. They're wanting a custom software solution today, but there's nothing to say they can't want a custom hardware solution tomorrow. Sure the enclave has some sort of lock out now but who is to say you can't simply reflash the firmware or perhaps just solder in some jumpers? Make no mistake. The FBI is wanting manufacturers to modify devices on demand.
2) If they beat the order then the FBI needs to find a new way to compel Apple to help them do shit. That likely means the FBI needs federal legislation passed, which in the current climate will buy Apple considerable time. This is why they want to beat the order (though the feds could further appeal all the way to the.... wait for it.... 8 judge supreme court!) Though I think Scalia would have be on our side on this one). It's not about this particular case since there will be others, it's not about this particular order since there will eventually be legislation.
3) So why go ahead and do it anyways? Naive (but still valid) reason: they happen to be able to help and without it being ordered they can do it without handing over a tool for ad hoc decryption. They can even go to Hawaii after and throw the dev machines into a volcano to satisfy their inner hobbit (I highly recommend this part of the plan) to ensure that no one can abuse the power of the one ring.
The non-naive answer is that for a quick project they get to show to the public that no, we the nerds are not so obsessed with abstract systems level thinking that we won't help when we can. It gets a lot harder for that moron Comey to hit the morning shows and throw shitty innuendo at the tech industry implying that we're aiding the terrorists.
Both encryption and terrorism are complicated subjects that are scary to the average American and although they distrust the government, they also distrust Silicon Valley. The cryptowars aren't about being right or they'd have stayed dead in the 90s where they belong. Basically Apple makes tech look like the good guy fighting terrorism, and for anyone who cares (smaller audience than the fighting terrorism bit) they also defended your civil liberties.
4) This trick only works on older devices. They will die out soon anyways. Newer devices are safe anyways. If they beat the order and do this one case voluntarily then no precedent is set, so they can't be bullied into doing it and old devices are safe.
One device compromised, all other devices safe, order beat, PR win, Comey looks like a prick even to the uninformed next time he insinuates that we're the enemy.