Section 2. Congress shall have power to enforce this article by appropriate legislation."
I'm sure the prisoners take the former interpretation that involuntary servitude was not specified as punishment and thus is unconstitutional.
Also, the 13th amendment absolutely bans slavery, so if you can construe characteristics of ownership then you could make it unconstitutional. Georgia also has a history of abusing the law to keep slavery in all but name going which would negatively impact the states' case in a non-southern court, and would make a good case as to the intent of Georgian legislation being to revive slavery. I mean when they're trying to put the Confederate flag back on their flag it gives a person pause as to the intent of not paying their labor. http://www.foxnews.com/story/0,2933,85078,00.html
This is why most states pay their employees in prison a pittance rather than absolutely nothing. It avoids the issue entirely and makes it one of labor law.
Re-education through labor, not just for communists anymore!
It is also for capitalists. Georgia has a number of privately operated, for profit prisons. (It does not appear that any of them were involved in this strike.)
[ http://www.dcor.state.ga.us/Divisions/Corrections/PrivatePri... ] [ http://www.thegeogroupinc.com/ ] [ http://www.correctionscorp.com/ ]