I understand the theory behind this, but I have to say that if a family member of mine were, say, brutally raped and murdered, I would be very uncomfortable with a 15 year sentence. Do the crime at 22, get out before your 40th birthday? It's one thing to say that there's no point in keeping someone in jail once they hit their 70s and aren't a practical threat to the health and well being of their fellow citizens, it's entirely another to release murderers while they are still young enough to be dangerous (presumably there are systems in place to reduce risk, but still.)
That's not even bringing up the important concepts of retributive justice and/or deterence.
Yes the United States has sentencing guidelines that are too harsh in many cases (Drugs being the most egregious example.) Yes we still have a terrible crime rate, despite these harsh sentences. Our legal system is deeply, deeply flawed in many ways. But I have to say that for a crime as serious as slaughtering almost 200 people in a terrorist act, I think life without parole is completely appropriate, and I don't think violent criminals deserve only a decade and a half behind bars.
It is interesting, though, that Europe's crime rate isn't sky high despite these leniant sentences. Kind of puts a damper on the deterence argument for those who want every pot smoker jailed for life, and everyone else executed.