Prison labor is extensive, and they make an absolute pittance for their work. The justification is that the state is providing room and board so they are just recovering costs.

But the other comment about prisons just replacing slavery is spot on. Former slaveowners were terrified of the prospect of rebellion, or worse, simply being outnumbered and losing political control with a simple vote. So they worked overtime to disenfranchise and repress former slaves through abuse of the legal system. Ripples of these measures still flow through society today.