You're missing the point. Apple sells The Downward Spiral in the iTunes store. Not allowing the same content in the App store is just absurd. In fact, the app doesn't even contain the content, it's just accessible through it.
I believe the reason for the apparent contradiction is this: Apple acts as a publicizer for the apps in the app store by letting them into the top-N lists and recent releases. When an "objectionable" app becomes popular, users hold Apple to account. They don't do this with the music store.
As the app store gets more and more crowded, Apple will most likely loosen up. Or maybe they will do the really intelligent thing: allow everything in, but only allow "non-objectionable" material into the top-N and recent releases sections.
Apple has a top-N list for songs, albums, audiobooks, movies, tv shows, etc. on the front page of the itunes store. How is advertising questionable material in those lists any different than advertising apps with questionable content?
With iTunes, Apple is a distributor. With the app store, it's a publisher. It's reasonable to assume that most people who are likely to get angry about content in a song/app recognise and understand the difference. And perhaps many of them don't understand that Apple is merely a publisher, and don't actually create most of the apps.
Creators and publishers are (usually) considered to have more responsibility than distributors when it comes to objectionable content. I think that's an important difference for Apple's image.
Well, considering most of the people here write software, and this has to do with a (currently) popular software platform, I think it's worth pointing out.
