Logically, slavery as punishment for crime is actually pretty reasonable and theoretically good. The criminal isn't just taken care of by the state, thus costing the people even more, instead, they actually have to pay for their crime by working it off.

But reality intrudes upon this theoretical situation. Since someone benefits from the criminal's work, there's now incentive to imprison as many people as possible. It creates perverse incentives that cannot possibly be avoided.

But almost as bad a perverse incentive is the for profit prison system, even if they aren't allowed to force prisoner labor. Because for profit prisons again have the incentive to imprison as many people as possible since that makes them more money; anything that reduces incarceration rate means less money for them.

Of course, we have both of these going for us. For profit prisons that make more money off the state the more prisoners they have, and the permission to force labor from them since the Constitution specifically allows it, thus letting the prisons make money twice off each prisoner. Yay!

In this context? Probably so that prisons pay for themselves. Or the loophole was intentional – it seems rather obvious that Southern states could pass arbitrary laws and enforce them willy-nilly, targeting minorities or whoever fell out of favor of the ruling class.