It's been firmly placed in Trek that it is position before rank and that a field promotion sticks, meaning if a Captain made an Ensign, a Commander in the field due to some circumstance, Star Fleet will leave the person at that rank because they trust in the judgement of the Captains, Which is how Kirk, even as a cadet, but given a field position as 1st Officer of the Enterprise is able to leap frog to command.
Everyone was put into their roles because of the situation. McCoy was made Chief Medical officer because he was taking over in medical when the orginal CMO was killed in the attack. Uhura was givin the communications position because she proved she was better than the guy that was there.
Spock was already a Commander by rank so there is no issue here.
Although I'm not sure what Trek examples about field promotions sticking you have, that's an interesting direction. I never noticed, and correct me if I'm wrong, but as for the crew of the Enterprise, did anyone get the impression that they were mostly cadets? I thought that they were supplementing a crew of officers already aboard, but they could have very well not even had a crew aboard other than a skeleton crew, since it was in spacedock. And they were scrambling a LOT of cadets, it looked like.
If they're all cadets, except for some key personnel (Chief Olsen, the CMO, etc), then it would be perfectly logical for Pike to appoint Kirk, who's a talented cadet, and more importantly, a senior cadet, to a First Officer position. And performing in that position, the feats that he did, he could have kept the equivalent ranking (If a first officer on a Constitution class is a Commander, usually), and got another uptick to Captain. And yes, he was a Captain in rank, not in position, due to the rank insignia. If you look at the trailer, and pause it before Kirk slaps Spock's back at the end, you'll see the insignia.
This makes it better for me. Not to say I was outraged, I mean, this is the same show that brought Spock back to life by shooting him into a planet being born that somehow reverted him back to a baby and made him back into a man without eating a bite. Just saying.