http://www.afscme.org/news/publications/privatization/pdf/AFSCME-Report_Making-A-Killing.pdf

Every year, America’s largest private prison companies – The GEO Group, Inc., Corrections
Corporation of America (CCA), and the Management & Training Corporation (MTC)—pour
hundreds of thousands of dollars into the campaigns of governors, state legislators, and judges, in the hopes
of advancing their political agenda—establishing more private prisons and reducing the number of public
ones. Despite significantly higher rates of inmate-on-guard assault, violence, and escapes in broad daylight
in private prisons than in public,[1] these companies’ strategy of pay-to-play has proven successful. A state
think tank in Ohio recently documented a 48 percent increase in private prison inmates between the year
2000 and 2009—leading almost 8 percent of incarcerated Americans to be housed in private prisons by the
end of the decade.[2]

http://government.cce.cornell.edu/doc/html/prisonsprivatization.htm

Those who oppose prison privatization make the case that the industry has the incentive and the wherewithal to extend the amount of time convicts will remain in prison, and that this presents a threat to justice. The industry, they say, can extend sentences in two ways. First, it has thrown its influence, through lobbying and campaign contributions, behind “tougher” laws such as "three strikes", mandatory minimum sentencing, and "truth in sentencing" that increase the duration of sentences. The conservative American Legislative Exchange Council (ALEC) has been extremely active in advocating truth-in-sentencing and three strikes policies throughout the United States. This organization is heavily funded by the corrections industry, and indeed ALEC's Criminal Justice Task Force is co-chaired by Brad Wiggins, a former director of business development for the Corrections Corporation of America (Bender, 2000). The strength of these kinds of political influence, opponents fear, will only increase as the industry grows. As one observer notes, corrections corporations have "paid handsomely to play the public policy game, and will likely do so again"(O'Connell, 2002).

The second way opponents of privatization worry that private firms will distort the administration of justice is by exerting undue influence on parole hearings. Opponents argue that since prison firms are generally paid per prisoner per day, they have an incentive to extend inmate stays as long as possible, and so are liable to reduce prisoner’s chances for parole or good time off by exaggerating or fabricating disciplinary infractions (DiIulio, 1990).

Industry supporters point out in response to these concerns that industry campaign contributions are smaller than those made by public sector unions ( Moore, 1998). There is no evidence, they say, of private prison officials manipulating parole decisions.