He should never have been down there endangering spock's life in the first place, and though morally it was the right thing, he did alter the natural development of the Nibiru native population, and broke Starfleet's #1 directive of not interference.
There is no further development period without the interference.
Yeah, Spock's preaching to Kirk about violating the prime directive made no sense, because Spock was doing the same thing by stopping the volcano!!
Like Pike said, they were there to OBSERVE.
The Prime Directive in TOS days seemed a bit flexible.
I bet if there was dilithium on that planet the Federation would allow interference.
In TOS they were gun-ho about saving the Amerind planet even though it was pre-Warp. Spock went to a lot of trouble to save that planet even putting Kirk at risk.
It was ridiculous of Starfleet to send the 'Enterprise' to 'observe' unless Starfleet really wanted to 'observe' the captain and crew.