I take no position on whether the books are pornographic--I haven't read them or investigated them at all.
But at least for young children, exposure to pornography is well-recognized as harmful. Not just morally, but psychologically. Maybe even as a form of child molestation.
For the same reason a 17 year old can't go to a movie theater and watch an R rated movie by themselves. Society has said certain aspects of life (violence, sex, language) need to be restricted until people reach a certain age.
I'm not confident that all kids could be like you, though. Even as an adult, I'd find it pretty disturbing if I were flipping through a book I liked, starting to identify with the main character, and then turned a page to see some side characters discussing their graphic sex fantasies about her.