So people are expected to know that and then trudge back up the stairs/escalator and walk three blocks uptown just so they can pay their fare? I've been to Manhattan often enough to know that those blocks are l-o-n-g.
If the NYC subway system really wants to save money on fare collectors, just do what the toll roads are doing and issue passengers those E-Z passes in the form of proximity HID cards. Just slip one of those into your wallet and the turnstiles will let you through as soon as you come up to them.
Street blocks are not long in Manhattan. Avenue blocks are. And unless the MTA’s suddenly gone more incompetent than normal, there would be signs indicating where unstaffed and staffed entrances are.