No, the "slavery" is while they are in prison/probation/parole. When they are released from said, they are no longer "slaves".
If a state (via the legislature) wanted to declare that a person convicted of a particular crime would be a slave for 100 years or his natural lifespan, but his only obligations as a slave were to (1) appear before a parole officer once every five years, and (2) not possess weapons, why would it not be within its authority to do so? Basically a very "loose" parole.