In the case of subscriptions (or any content), which this is, would you expect to go to a store, say Sears, and have people selling things inside of it without giving Sears a cut? How is it that selling apps in a virtual store is somehow magically different?
When we're talking about in-app content that is not distributed by Apple (i.e. Netflix movies, Kindle books), Apple is not doing anything to earn a cut. They provider is providing the hosting, the user is paying for the bandwidth (be it wi-fi or 3G), and Apple is doing nothing. A better analogy is the manufacturer of the Sears cash register demanding a cut of the items rung up with it.
Of course, it's true that Apple is providing the hosting for the original app itself, which is often free. But Apple is getting their cut anyway, through the annual cost of the developer program, which is required to have the app in the store.