Do you really not see how free labor gives the state an incentive to lock up as many people as possible? That's absolutely modern day slavery. Adding, "Yeah, but these guys deserve it" doesn't change that.
More, we should want our prisons to be structured in such a way that when people get out, they have a chance to have a decent go of it. If they're paid and allowed to accrue money so that they've got some cash when they get out, they're much more likely to have a better time of it than if they're just dumped outside the gate with the clothes they wore in.
Green Line is only concerned about the punishment aspect of incarceration. From what I can tell, I'm not sure he really cares about recidivism or anything of the sort. At least that is what it seems like, based on the back-and-forths I've had with him on this in the past. I know there is a personal story somewhere behind his deep contempt for prisoners/criminals. I just know it.