I don't think that's allowed, Or they can put another category, That's just the site discretion of maybe if the majority of readers like that kind of book to read.
I think they can categorize that and they can filter it from all applications. I'm sure some of people really like to read that kind of stories. Gil just only need to put some cautions or any warnings before accessing it or only 18+ members can access it.
Yes you, that, definitely there should be a distribution of books on such a classification. Otherwise, should not be, absolutely everyone will have access, adults, students, children. I'm sure that the developers should have thought about this.