Slave labor is the typical sentence for violent criminals in my homeland, as well. However, we don't need mining or roadworks or any of the usual assignments.

I'm not sure we have the resources to provide for the welfare of a prisoner. It isn't simply care and feeding. There's also the monitoring necessary to insure that the prisoner isn't abused. To my knowledge, we don't have any qualified personnel for guarding a prisoner long term. It is slightly more complex than, "Off you go, try not to hit the prisoner too often."