How is life in prison without the possibility of parole not, in fact, the death penalty?-- taking away their liberty and confining them till they're dead.
However, I will be presumptuous and say you would argue that if evidence can be found that would show the conviction to be a mistake, the convicted person could be freed and have years of life. It's also true that escapes are possible with 'life without parole,' and not with more immediate execution (rather obviously). Consider the case in Fort Worth a few years ago, where the escaped convicts murdered a man.
And while virtually no one would really advocate the above example of each prisoner having his own hole, to be shot immediately if he so much as raises his head out of it, more work should be gotten out of them. But in Arizona, where that sheriff put them on the chain gang doing road construction, people see them and they get all that sympthay! The 'punishment' for refusing to respect the life, liberty, and property of others is to get free food and shelter and do nothing to earn it. Make them work, pay them in some form, and let them buy their food, blankets, privileges, et al.