Apple is about as respectable a moral gatekeeper as Wal-Mart with a somewhat wealthier customer base; but I suspect that there is a fairly logical and pragmatic reason for their distinction between ‘apps’ and books/music/movies:
With an ‘app’, odds are pretty decent that it’s iOS only(or at least iOS first with some other platforms playing catch-up), since iOS has a pretty commanding position in the mobile applications market, especially for heavyweight stuff that makes hardware consistency valuable or where the asking price is comparatively high. Even if it isn’t a platform exclusive, there is no non-Apple source for iOS apps: If an app is in the app store, Apple Inc. is directly responsible for that and has only the weaksauce must-have-been-a-pesky-peon-acting-contrary-to-policy excuse.
For movies, music, and books; Apple sells them; but it is both reasonably easy to buy them from non-Apple sources and use them on iDevices; and all of those markets have assorted non-Apple entities who have become widely accepted as being the ratings dealers or the moral gatekeepers. It’s much easier for Apple to simply say that “We offer the same selection as comparable retailers; and support filtering music flagged with a Tipper sticker and sorting movies by MPAA rating; don’t blame us, blame The Market.” without being especially weaselly. It’s largely true that Apple just carries the same stock, minus any exclusives that aren’t theirs, as anyone else; and they do offer tools for moral-arbiter filtering; and if you can’t get it in the Apple store you can get it from Amazon.
There is the ESRB for video games; but (while it has staved off the worst of the kneejerk reactions) it has limited traction(People typically refrain from taking small children to R-rated films and then crying about how inappropriate the content was; but games rated Teen or Mature get plunked in front of kids all the time, and complaints about how age-inappropriate the brutal-but-clearly-marked murder simulators are are nearly as common) and the substantial majority of mobile titles are too small scale to deal with the ESRB process and are never actually rated.
None of this makes Apple a good gatekeeper; but I think that Apple’s reasons for playing gatekeeper in software while (mostly, I’m not sure that they actively stock the finest in National Socialist Black Metal; but their catalog is a comparatively unmodified passthrough of whatever they can license) washing their hands of other media are logical enough.
