I imagine the librarian was acting on a supposed (imagined) community standard about what parents want their kids reading-- maturity as well as difficulty.
Agree with AEH-- my kids really do put books they encounter down if the content is beyond what they want to process emotionally right then. I imagine that not all kids do, so those families might need explicit rules. We haven't needed them.
(And I would despair of enforcing that sort of rules around here in any case... the kids will read what they find...)
