Because forced labor isn't recompense. It's torture for profit. They can contribute by learning and growing as people who can then get out and lead legally productive lives. The purpose of the system shouldn't be revenge or exploitation. It should be to create people we want back in society. But we can't get over this idea of punishment, wherein we can just declare somebody a criminal to be punished. For example with marijuana. They could be a perfectly fine person, but God forbid they have the plant equivalent to alcohol around. There's no rehabilitation needed there so all we do is punish them and then suddenly we've created a need for psychological counseling and job help.