Privitizing prisons is a bad idea. That makes the prison a business. What do businesses typically do? Increase profits, layoff workers, and look for ways to cut corners while increasing the customer base. What does this mean if a prison is a business? If companies make money from running prisons, obviously they will want more and more prisoners, and will lobby for more laws and stronger punishments. America already has the largest prison population in the entire world. The idea of executives plotting to increase prison populations, and hence their bankroll, should make us all queasy.
Prison should not be a place to cut corners. Instead of locking people up and forgetting about them, we should be looking at ways to make inmates productive members of society so they can assimilate after release, instead of being career criminals. Programs that allow prisoners to train dogs have been very successful, and enjoy public support thanks to portrayal on TV. Will those programs continue in a commercial prison, whose goal to make money? Not likely.
Safety is also a concern. If commercial prisons cut staff, that means both employees and inmates are less safe. A report on NOW (http://www.pbs.org/now/shows/419/video.html) shows that privitized prisons have more security problems, including riots.