Politics drives it. In the US, no politician dares look soft on crime. Advocating ridiculously long sentences is a quick and easy way to bolster an image. And failing to be tough is an even quicker way to end a political career. Huckabee is getting flak because one man he let out early has shot and killed 4 police officers. Type "Dukakis" into a search engine and one of the first things that shows up is Willie Horton.
You are half right. Looking tough on crime never hurts, on the other hand, most people would agree that unsupervised furloughs for people spending life in prison for murder may not be the best idea(thus William Horton).