luchog
Neo-Post-Retro-Revivalist
Because there is still the chance that he may, eventually, through some quirk of politics, be released. Charles Manson still comes up for parole every few years; and his application for parole must be formally refused each time. There's also the risk of escape. Squeaky Fromme escaped briefly in 1987. They are also often threats to other inmates, and numerous mass murderers have been known to kill or assault other inmates. There simply aren't sufficient resources currently to keep them warehoused individually (although there probably would be if the government would stop locking up non-violent drug offenders).I cannot see how someone faced with life imprisonment is a threat to society, let alone a dangerous threat.
And there's the issue that personality-cult leaders like Manson could still provide inspiration and guidance while imprisoned; or prove to be the focus of terrorist activities (like some Irish liberation group leaders were). Yes, there is the risk of making martyrs; but for personality cults, martyrs are far less dangerous than live leaders. Without the influence of a suitably charismatic leader, such cults typically disintigrate due to internal conflicts.
That said, I do not support Capital Punishment in practice, although I do support it in principle. The statistical likelihood of convicting and executing an innocent man are still unacceptably high. Unfortunately, the alternative isn't substantially better.