You think most app buyers found their apps outside the app store? Unlike folks reading HN or tech sites and trying new apps thanks to app launch news, the normals I know find their apps only within the app store (other than the occasional app roundup in a mainstream printed magazine).
If you look at the CC processing industry, ranging from 5% fees for the most basic service (transaction + cc + merchant + bank) to 15% fees for an end to end service that cuts you a check, the 30% to also manage the user authentication and customer service is a reasonable deal, completely aside from marketing.
Finally, as a user, I don't want featured apps to be the ones paying to be featured. Not cool.
We are talking about different business. apps/ebooks generally don't have a high margin.
Apple has a cost of no more than 2.5% for credit card fees, and its probably even a few tenths lower than that. That I will guarantee (seeing as how I have seen companies in the $5million/year range get that percentage from an end-to-end provider, and Apple is quite a bit more than that in volume).
What I think is really ridiculous is that they want to charge 30% for subscription type services. One time app purchases... ok, maybe I can see that. But for a $5/month or more subscription being bought through there its absurd and making it more of a pain for the user because the app developer is forced to break with the single-billing system due to costs. So, simplicity actually hinders the experience here.
My company has developed a CC gateway server, and has processed as much as $80M a year in transactions for a single client. This is a business I've spent a lot of time in.
Also note that digital goods and physical goods are different categories. A full service provider of "digital goods only" processing for clients without merchant accounts charges much more than a company like Paypal, Braintree, or Stripe, which process for physical goods as well.
I have never understood this argument. It's a bit like saying "WalMart does a bad job promoting milk discovery, so I think they should sell it at cost."