Companies and/or websites can ban almost any speech or writing they would like. It's not unconstitutional, because corporations aren't considered a governmental/Congressional body.
But wanted to highlight that the law can ban speech, despite the First Amendment. For example, in 1982 the Supreme Court ruled unanimously that child porn was not protected by the First Amendment:
http://en.wikipedia.org/wiki/New_York_v._Ferber
Laws can also ban material deemed obscene. The Miller test is the standard there: http://en.wikipedia.org/wiki/Miller_v._California
What about Adrian Lyne's 1997 movie, Lolita? It has artistic merit, but arguably not as much as the novel.
What about girls' dancing and gymnastics? Should photography of those activities, even including women's international gymnastics competitions, be banned just because, viewed with an eye toward prurience, they might qualify? Certainly the activities have artistic value, but how much?
What about purely fictional depictions of minors, either in manga (see the case of Christopher Handley's manga possession) or even in fictional stories that may not be deemed to have serious literary merit?
It would be pretty unfortunate if literature couldn't even seriously deal with the subject! (Incidentally, it's pretty hard to read Lolita as being pro-pedophilia; if anything, it's an extended, mostly negative investigation into the protagonist's psyche, although many pages in literary journals have been spent arguing over how to interpret it.)