This is straight-up incorrect grammar, and I don’t know of any style guides that would condone this – let alone mandate it.
It’s an obvious typo; my pet theory for the most likely explanation is that it was originally “Libraries turning into drug, sex dens”, but then, at the last minute, somebody changed it to “drug-infested sex dens” and forgot to remove the comma.
This is straight-up incorrect grammar, and I don’t know of any style guides that would condone this – let alone mandate it.
It’s an obvious typo; my pet theory for the most likely explanation is that it was originally “Libraries turning into drug, sex dens”, but then, at the last minute, somebody changed it to “drug-infested sex dens” and forgot to remove the comma.
Perhaps they’re dens of drug-infested sex?