The morality of time travel "fixes" and the judgment of Spock and everyone else in Trek is always conveniently tailored to the plot of the episode, be it "City On The Edge Of Forever," "All Our Yesterdays " or "The Voyage Home." The writers in this case set things up so that it's not within Spock's power to do anything about cosmic events - that's all. There's no point in citing Spock's past "character" as an excuse to claim that Spock isn't actually Spock.
Spock Prime is Spock from TOS because Nimoy and everyone associated with the movie say that he is. They get the first, last and only word on what's an actual part of Trek "canon" and what's just folks blowing smoke on a BBS.
I know. I just enjoy defending Spock Prime's actions to closed-minded TOS fans.
Playing time police raises some interesting moral questions, beyond the shallow "let's fix the timeline!", too.