Cynically, I'd say Apple is (successfully, judging by the commentary here) solving the problem of finding reasonable-sounding justifications of their app store approval process (and the inevitable arbitrary decisions that come with it).

They're trying to avoid the situation where they have to stand up to a morality-in-the-media-type group and defend someone else's freedom of expression.

Allowing the web is PR-defensible: "it's the web". But when it comes to things you explicitly review and choose to allow in your store, you're forced to either stand up for adults' right to that content, or you preemptively censor yourself and your suppliers. (a la Walmart)