This is not a new problem in US entertainment and media. Compare the behavior of Apple with the MPAA ratings board which treats violence and sex in a similar manner, rating films which depict violence with an "R" while effectively censoring films with certain kinds of sex scenes with "NC-17" ratings (in case you aren't familiar: films rated "R" will be shown in most movie theaters but many if not most theaters choose not to show "NC-17" films). See the documentary "This Film Is Not Yet Rated" for more.

I have no idea where this comes from, if it stems from some kind of deep rooted puritanical culture, but it's not exactly unique to Apple's app store.