I'm guessing there is tons of complaints if a regular store denies a product from their shelves too? And an uproar if Steam (or any similar service) denies a game from being distributed using their system? I also assume that the same protests occur if, let's say, Bodega (or Chrome Web store for that matter) refuses to list a specific app?
There will be if that store starts putting its storefront right in every house and apartment. (Not commenting on the legitimacy of the complaints — but monopolies do tend to get a lot of criticism when they're perceived as being unfair, and software bundled with the computer comes very close to that threshold.)
There is a bit of a psychological difference. People creating Mac Apps are for the most part Apple's customers and many identify with the brand. So the rejection hurts a bit more.
