Prisoners should be a tax burden. That way, there's no incentive to conveniently round up more prisoners every time the local town needs more free labor.

http://theconversation.com/exploiting-black-labor-after-the-...

> The living conditions of leased convicts are documented in dozens of detailed, firsthand reports spanning decades and covering many states. In 1883, Blackmon writes, Alabama prison inspector Reginald Dawson described leased convicts in one mine being held on trivial charges, in “desperate,” “miserable” conditions, poorly fed, clothed, and “unnecessarily chained and shackled.” He described the “appalling number of deaths” and “appalling numbers of maimed and disabled men” held by various forced-labor entrepreneurs spanning the entire state.