The astroturfing relies mostly on anonymous users. The vast majority of trolling and shilling on Twitter and similar platforms is done with fake identities. So you have a few open shills who are using their real names, with massive campaigns enabled by anonymous/fake users
What part of that requires anonymity? You pay some broke college students or unemployed dog washers to shill (or let someone else shill) for the big accounts under their name.
There is not only a massive supply of such people, they have high turnover as the seniors graduate but the new freshmen are broke again and the unemployment rate is fairly stable but the specific people distressed enough to sign their name for a buck are constantly in flux, so it doesn't even matter if they get banned.
