I'm pretty sure the M comes not just from the violence-in-combat, but also from the piles-o-corpses-and-skeletons set decoration, cannibals, and "mature themes" (i.e, log entries describing living through the bombing and then dealing with everyone around you dying of rad sickness).
And that's ignoring the silly stuff that the rating people would be considering, like "omg, drugs" or "you can drink and get drunk!".