Theory and practice are separate in America's case. We're not rehabilitating criminals as much as we're punishing the poor and disenfranchised for being such. The majority of prisoners aren't doing any prison labor at all, because slavery is the point first and foremost. We're stripping people of freedom, reducing them to less than human, removing opportunities to become more than the bottom caste of society, and telling the country its their fault. And then a very small percentage of that population is given the privilege of working for minimum wage, which I feel says a lot about how awful prison conditions are when we consider how embarrassing minimum wage work is in America.

This is very far from an ideal system, and we seriously need to rethink the purpose of prisons here before we attempt to make any arguments that slave labor is a good thing, if only for a fact that it is still slave labor and not rehabilitative work. Otherwise, what you're really saying is "our slaves ought to work to prove they're capable of being working slaves, as opposed to the subhuman trash we've determined them to inherently be". We've had this very same conversation before, and it was just as fucked up the first time.