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.