I don't feel like anyone did anything wrong here. the library probably had no clue what was in the book, they can't read everything that comes through their doors. the girl likely thought it was just a romance manga with cute boys. Dad (or whatever relation he is) saw this, decided that it contained inappropriate content for the girl, returned the book, and lodged a complaint. The library then responds with it's policy that prevents the librarians from keeping certain material out of peoples hands. Hopefully dad will discuss the 18+ sticker with the girl and why she should not check out such things and the library will rethink it's shelving arrangement.