I can't understand why you think this is a good idea then. Prison should focus on one thing and that is rehabilitation. Not exploitation. The idea that "society" can be owed something from these people is wrong. It's just exploitation. The objective should be get them on the right path and get them out of government care, not keep them and force them to pay some imagined societal debt. If this becomes common place there will be to much incentive to keep prisoners. Look at the sheriff bragging how much he saved the government by using his prisoners.