I was.. precocious as a youngster and read books that were far above my grade level and what most adults would consider to be "safe" for children.

The first time I tried to check out one of those very adult books the librarian called my parents and asked if it was OK. My parents said "Yes. Let him have whatever he wants." They made a note in my account and the next day they let me have have whatever I wanted.

If that hadn't happened I would be a very different, and much dumber, person now.

I don't understand what the issue is with just asking the parents?

I suspect that most of the people responsible for these "bans" don't want that to happen because some parents will approve of things they don't. Most of this really IS an attempted ban rather than just "appropriate age related content" issue. They don't want to control what THEIR kids can see. They want to control what YOUR kids can see.