Humorous commentary makes the case that for-profit prisons have no interest in stopping crime, they would rather have people behind bars. Since the 1980s violent crime has gone down considerably, but mandatory sentencing for drug violations has placed many nonviolent criminals behind bars for extended periods of time. After a lengthy stay in prison is hard to return to society as a productive citizen.
Public Prison guard unions routinely lobby for campaigns and laws that benefit them. For example, they spent millions opposing marijuana legalization. https://theintercept.com/2016/05/18/ca-marijuana-measure/