Seems straight forward to me
Seems pretty straight forward to me. Apple charges for In-App-Purchases of Digital content that would/could be used in conjunction with the app.
The reason is clear, Apple HOSTS the app and the entire App store environment, to pay for this, Apple could charge a "flat" fee on every app download, but that would be punitive to "cheap" apps and prohibit free apps. Sooooo..... Apple charges a percentage of the purchase price, 30%. This allows for free apps (no charge), Cheap apps (small charge), and expensive apps (big charge). If Digital IAP were allowed to bypass Apple's store then you could release a "$10" game, on the app store as a disabled "freeware"(i.e. play for 1 minute or 1st level or something) for free, therefore not pay for the hosting of the app, or the search features that found the app, or the bandwidth to deliver the app, or the security development and implementation so securely deliver your app, etc... Then inside of the free disabled app the user would enter their credit card and buy an unlock code for the full game.
If this In-App-purchase could circumvent the app store then many, if not all, "Pay" apps would eventually go this route,as Developers would get more profit (By getting the hosting for nothing) and this would deprive the App store of the needed income to run the store.
This sort of pricing policy would never fly in the outside world. For example, We would never allow a manufacturer to “sell” a product for $0 in best buy (taking up shelf space, sales force etc…) just to charge the “Actual” price directly to the consumer to “unlock” the product. Nor would we fault best buy for demanding a percentage of the “real” value of the product.
I know that is a sloppy analogy, because there isn’t a really good physical analogy to the digital world
But, if IAPs are allowed to bypass the app store, then it will be the beginning of the end for the app store.