There are around 2 million people in prison within the United states that are on record ( not counting military detention centers and so on) It cost about 23,000 dollars per year per prisoner, according to a 2001 report and that doesn't include whatever health care they are given, maintenance, material, food and all those knick-knacks.
The money has to come from somewhere. The system isn't perfect but it does cost to maintain it. Times are tough.
http://www.quora.com/How-much-money-...-cost-per-year
http://usgovinfo.about.com/cs/census...aprisonpop.htm