I don't disagree, but I don't really know much about the situation at all. Never heard of the game, never saw the trailer. It sounds like it wasn't anything too bad, so that seems strange.
The main thing I'm thinking is that they have little motive to do anything nefarious with this. All the ESRB has to gain from somehow convincing them to pull their ad is less controversy in the mainstream press and their own continued existence. It's not like there's really any benefits associated with the 'authority' they have or any chance of financial gain for them from this. And as far as I'm concerned, their motivations are directly in line with ours: Controversy = bad, self-regulation = better than external regulation. It's not that I'm goose stepping here, it's just that we don't know much about the actual situation it seems and looking at it logically it doesn't seem like any sort of abuse of 'power'.