Not american so someone correct me if i'm wrong, but from videos I've watched prisons are only obliged to provide you with the minimum average male calories per day so around 2000, for some people this isn't going to cut it, especially if you want to work out, and considering how little there is to do why wouldn't you?

Many activities are now charged as well, some prisons brought in tablets instead of having books, these cost money every day you rent them. Phone calls cost money.

So it's not exactly forced but if you want to do anything other than sit in your bunk all day or watch the tv's you are going to need some form of income, be it prison work, family, or a prison hustle of some variety.

I don't think there is anywhere that literally forces you to work. But I think you basically have to in many cases. In a lot of prisons, basic necessities like hygiene products cost a crazy amount of money on an extremely small "wage". In situations like that, they might use 13th amendment to argue that it is legal (because slavery is legal), but I don't know any instances of that happening.