I think it hinges on whether the private prison industry is lobbying for more funding per prisoner or tougher laws that result in more prisoners (or longer sentences). If they argue for safer prisons, better rehabilitation programs, more security for guards -- the public should be skeptical but open. If they want to just lock more people up and then bill the government -- that would be somehow like teachers' unions lobbying for less birth control to influence the birth rate and therefore create more demand for teaching jobs. It seems perverse.
In other words, we need prisons just like we need schools, even if people like schools more we shouldn't vilify corrections officers just because they work in an industry we don't care to interact with. It's all about their conduct, which has been terrible. Teachers unions are also self-serving, but they at least pretend to have the child's interest at heart.