If you're robbing a place and your punishment for being caught is effectively life in prison, what reason do you have not to murder all the potential witnesses to improve your odds of getting away with the crime? All you have is morality, and I don't want to rely on the morality of criminals.
I know that the deterrent effect of prison on criminals is highly debatable, but if somebody is robbing me, I don't want all of the incentives given to him to point towards murdering me too.