Because some fanfic is "dark" or "pornographic." Apparently reading fiction corrupts girls' morals and makes them sulk and spit. This is related to right-wing scaremongering about the internet, a discourse that today tends to focus on "the internet will turn your child trans." Much as Joyce may claim that liberals and leftists can be anti-trans too, pragmatically she's aligned with old conservative religious principles of Don't Let Our Daughters Read Books.