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.)
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?
There is a lot of fiction from the perspective of non-sympathetic characters. If doesn't mean that those who appreciate the insight the fiction provides into the character, or the character's circumstance, are advocates of the character or the circumstance itself.
Besides, no actual child was abused in the production of these works, unlike with films and pictures with actual underage models; and potential harm to future children is a rather shaky basis to justify flushing the First Amendment down the drain. That's the argument that ACLU's Nadine Strossen used two decades ago against censorship of (non-child) pornography. I see no reason why the same argument won't apply to child pornographic literature.
But private websites are a different matter, and there's nothing wrong with a moderator who chooses whether or not to discriminate on the basis of perceived literary quality on his or her own site. Many forums ban discussions about religion and politics altogether, but that doesn't infringe anybody's Constitutional rights.
> I confused about why he thinks it ironic
> that Nabakov is banned...