Apple isn't really incurring any costs for this. They don't make any of the content, they don't write any of the software, they do host the 'applications', but they only deliver the tiniest parts of the app (they host a small binary, but they aren't hosting the videos on Netflix or YouTube, for example, or the major assets in an MMO game or such) and a lot of the labor they do "provide" isn't necessarily valuable (their "review" of the apps, for example).
The 30% figure seems to be pulled from gaming console markets, but it was never really justified over there either, and the previous economics that made it work (like subsidized hardware) never applied to Apple's ecosystem. (iPads aren't like, $99 each, and priced at that point, with the expectation that Apple will 'make it up' via the 30% tax on app store sales, the way consoles once were)