The obvious, and yet rarely acknowledged, problem with private prisons is that their obvious market objective is to imprison as many people as possible and then profit off of them:
With their growing influence, this heavily financed lobby has also been able to slowly erode restrictions on the use of prisoners as commercial employees. Prison labor has long been banned in various states from competing on the free market since it violates numerous labor laws and essentially amounts to a slave workforce who can be paid subminimum wages and have little recourse against harsh working conditions. The for-profit prison industry is determined to change that.
This profiteering might be defended as part and parcel of running an efficient penitentiary, but it’s hard not to view it as a vicious cycle of exploitation; prisoners are used as cheap labor, sometimes against their will, obstructed from leaving in due time, and given worse treatment all to help fund a lobby that seeks to trap ever more into their galley. When the venture is not profitable enough, the inventory can be auctioned off to the lowest bidder like chattel, creating a kind of de facto system of legitimized slavery.