The difference is largely in the visual depictions found on that site. I'm going to quote from the response we give to complaints about stories with under-18 fictional characters:
We follow a policy of no censorship, as long as the content is legal. While we can't allow stories involving real persons under the age of 18, fictional characters are permissible as long as the story is properly tagged so those readers who prefer not to see such content can click on past it.
And given the volume of Harry Potter fictions involving a large cast of generally underage characters involved with each other, with professors, with parents, with siblings, and with creatures, I'd say that it's not so much the content as it is the context.