In 40-some-odd years of Trek, I have never seen anything to suggest that Spock would let the inhabitants of Vulcan die at the hands of a psychotic time-traveler and not try to undo that tragedy. There's no way that "Spock Prime" is the same person that Roddenberry created all those years ago. The only logical explanation, barring brainwashing of Spock Prime to make him a callous asshole, is that he is from an alternate 24th century, and he traveled back to an alternate 23rd. There you go......mischief managed! That way, dumbass engineers can build a gigantic non-aerodynamic spaceship on the ground, Chekov can be born any old time he wants, and cadets can be given permanent command of starships.