Specifically (we can only guess at the particulars as Apple does not go into great detail), Apple has a problem with the lyrics of a song called The Downward Spiral - which the app makes available via podcast. This is confusing - as Trent explains on the NIN forums - because the individual song and album of the same name are available for purchase through iTunes. Not to mention that the app features forums where registered users are free to type whatever they like - that aspect of the program didn't bother Apple too much during the approval process.
Thankfully, Mr. Reznor has clarified the dilemma for Apple's befuddled gatekeepers by demonstrating what unspeakable blasphemies their own Safari is capable of - on a phone no less. Adults and mature youths who can handle a couple of dirty words and somewhat graphic imagery can survey his evaluation of the situation and enjoy a sensible retort by clicking here.
Honestly, I can see how it would be tough to sort through so many apps for QC purposes - but the contradiction is difficult to ignore. It's good for a laugh, but if it were my App and I had gone through everything Apple requires to achieve approval (let alone what it took to create such high-quality software) I'd be pissed too.