I wish people would get off the 30% is soooo much kick. It's not. Get a clue, retailers and any distribution channel makes money when distributing and selling products and 30% is very common and fair.
But... the part that Apple will or should change is the part about having to offer the in-App price at the same or lower price. This is the part that publishers don't like and the anti-trust part as it's controlling prices.
If Apple changed that part, all would be good and let the buyer make up their mind as to where to buy.
You are referring to the classic distribution of physical items. This is digital - a new scenario - new distribution methods. Digital and physical distribution methods are NOT the same.
Apple want a 30% cut just because an application is hosted on their AppStore... essential for doing absolutely nothing. The publisher has already paid for the application to be hosted on the AppStore - why should Apple also take a cut of further purchases via that application? Why should any in app purchases be tied to Apple?
( Any other appstores that do the same as Apple, such as Google, are equally wrong , IMO )