That's certainly one opinion. What is or isn't is very subjective and not the opinion of all parents who don't wish their children to read certain books.

If there's any concern that a book may not be right for a child then it's safer to keep it from view.

Like I say it's a very subjective view and one for others to decide.

But it's certainly not a ban by definition nor in practise. It's a choice and many libraries will make different decisions and are free to do so.