The prisons are a service for the free people to have safety, they should paid by them. It is insidious to make prisoners pay for the prison, they are not the recipients of the service, its just adding a layer of oppression.

But if you dont like a moral argument, make it an economic one. Each prisoner in california costs like 100k a year: if you paid them reasonable salaries, and they reduce re-incarceration rates you will be saving orders of magnitude more money.