If we're gonna have prisons at all ...
It makes a great deal of sense to offer prisoners pay for doing things to maintain the prison: cleaning the floors, washing the uniforms, leading the singing circle or whatever. That sets them up as members of a self-supporting unit, where you can actually be rewarded for doing things that benefit the other people around you. Then when they get out, they're accustomed to being a person who makes things better for those around them.
But it doesn't make sense to put the prisoners out into the non-prison world as competition for free workers, and then claw back their wages. That sets prisoners up as underclass members of general society which is exactly the condition that leads to a lot of people becoming criminals in the first place. And then when they get out, they've already been "out" as slaves of McDonald's, so that's how they and the world are accustomed to relating to each other.
ⓘ This llama is extremely fuzzy.