Of course it's in Apple's interests to control prices, the question is whether it's in the public's interests. I'm undecided.

Apple can always control prices by being the exclusive seller of their products, that would be a valid way to do it. By selling it to a third party they, I think (again, am undecided) they give up the right to say how much it can be sold for. In the same way they can't stop you from selling your iPhone to a friend for cheap.

What they can do is not to sell to retailers who they feel are damaging their brand. In effect, this is exactly what they are doing, however they are doing it through written agreements rather than through observation.