Paying 30% to get access to millions of users, with money and having distribution, backups, creditcard handling, user management etc. Handled for you.. seems like a pretty good deal.
It's been shown over and over again, if you're going to build a mobile app and want to make a profit, you build it for iOS first, then if it isn't a huge cost sink, also make it for Android. The people most likely to pay well are iOS users. Why is that? Anybody against Apple will just call the users stupid, but if you really analyze it, most users believe that Apple provides a better platform, better app selection on their app store, etc.
There's clearly still an issue between "Apple wants to protect its customers" and "Apple wants to _protect_ its customers".
The app store fee is a "roll-up" of numerous fees including transaction costs, cost of running the store, and license fees for use of Apple intellectual property (all that Apple code that you import into your iOS app when you hit compile).
What Apple are actually charging for is the first item in your list - access to the users. Apple wired the hardware to make themselves a middleman in every transaction. That's it, that's all it boils down to.
The visibility and access to the user base that the App Store provides is a valid incentive, depending on the product.