"Dear Doctor" was disgusting. The future is just possibilities, that's just a BS excuse for inaction. If they become Space Nazis, deal with them THEN not now.
And of course, stopping the Volcano was not part of their mission. They were sent to survey they planet, not alter it's destiny. Kirk and Spock have morals and could not allow innocents to die, dumb regulations be damned. If "an experienced Starfleet crew" would have stood by and watched lives end for no good reason, then the universe is far better off in the hands of these young 'uns.