They're dominating the market, but it's not a monopoly (any more, now that there are more competitors).
If you play the game, you have to play by the rules. Nobody forces anybody to use the AppStore. It's the developer's decision (I know because I do play this game). But if you want to distribute your app through it you have to obey to Apple's rules, it's their playground.
You can always use Cydia instead or serve a different platform (Android, Nokia, RIM, WP7) if you don't like Apple or the AppStore.
Tourist, Rincewind decided, meant "idiot".
— Terry Pratchett, The Colour of Magic
Any sufficiently advanced bug is indistinguishable from a feature. -- Rich Kulawiec