Even though the pool operators have large discretion, the computational power that makes up their pools can be moved elsewhere... a little like a transferable proxy. Most smaller miners probably trust the pools who they've grown confident in... but a large enough controversy or campaign could conceivably shift the mining-power away from pool operators who make unpopular decisions.
Open source works the same way. Of course there are authorities for a given project who can choose what the final release is, but if they get too full of themselves or something it's easy enough to bypass them. It's happened quite frequently, so it's not an idle threat, either. You can't prevent authority from developing, that's wouldn't even be a good thing, but you can prevent it from abusing its current position of authority to lock in future authority.
They don't (can't) rule with an iron fist, but they have a lot of good will with the community, so most will listen to them.
EDIT- The good/bad news is that since most mining (and thus transaction verification) has consolidated into large pools of machines, only a few people have to agree with "them". So in a sense, there is a central committee of people who run the large mining pools that does have a fair amount of power.