Remember, prison guards get special bonus' for managing inmates that make the prison money. Now imagine that you are constitutionally a slave (according to the Thirteenth Amendment) and that your prison guard's bonus is tied to your work. You don't get to say no to extra shifts. You don't get to take sick days. You don't get to stop the line (for those making physical products). You don't get to challenge the safety of your workstation (for those making physical products).
I'm not saying these programs shouldn't exist, but you need actual safeguards to prevent the current rampant abuse of prisoners (at least on the UNICOR side). Guards should never be 'special UNICOR employees' tied to the program (they really start to see inmates as slaves, their job only exists as long as their facility's UNICOR program is 'successful') and should never have bonus' tied to inmate work output. Currently both of these things occur.