I wouldn't really consider prison labor "voluntary."

Yes, technically one could choose solitary confinement, loss of even what few privileges one has as an inmate, or never be allowed to see one's family again, but that's like putting a gun to someone's head and claiming they "voluntarily" signed a contract because they could just as well have chosen to have their brains splattered across the wall.

Plus, it is explicitly a continuation of slavery per the Thirteenth Amendment.