Apple brought you the customers, Apple adevertised your application, Apple provided you with the bandwidth to distribute your software and updates, Apple provided you with the SDK/APIs to write the application, Apple provided you with support for that SDK and the OS it runs on, and Apple takes on some legal liability for selling it.
How are they not entitled to something?
You wouldn't have the customers if they hadn't provided the market place.
This (and don't forget apple made the device itself).
Basically Apple is like a giant cruise ship company that can stop at any of thousands of islands along it's way. The islands that don't want to follow their rules (give us 30% of your bananas, don't use our private radio channel, wear some pants for god's sake, etc.) well that's their prerogative but they won't be able to sell their wares to Apple's thousands of well heeled tourists.
How are they not entitled to something?
You wouldn't have the customers if they hadn't provided the market place.