There are certain crimes against societies that have been deemed so abhorrent that we feel the need to permanently remove these criminals from our society. Murder being one, child molestation and rape being just two others. Extend the death penalty to be allowed in cases where a life sentence is a common outcome.
When someone has shown themselves to be so completely incapable of peaceably coexisting with the rest of society that we feel the person needs to be permanently removed from it (Such as a life sentence or other crimes against children.) it is unfair to the people to force them to pay for someone's care who we have removed as a member of our society.