Just to put a slightly different spin on this... The U.S. penal system has a fundamental problem: it's for-profit nature. Prisons create money and jobs, which create lobby groups and, occasionally, bribes for crooked judges (e.g. the "Kids for Cash" scandal). The incentives for this system are all messed up. For example, a prison that effectively rehabilitates 100% of its inmates would get fewer repeat "clients".

If the death penalty offers no deterrence and costs more, instead of less, why is anyone still using it? It seems like something certain lobbyists would like, but which actually runs against the public interest.