Its not the private prisons who decide to send people to prisons. Its actual judges following local, state or federal laws. Lets not pretend there is no judicial system...
That's not the norm though. The norm is self-righteous bureaucrats, who are worried they will be seen as weak on crime, or actually want to be even harsher than their constituents. There's scores of judges and legislators that feel this way. Most of the rest of them don't have the guts to stand up to them.