Probably when it's the parents making the decision for their own kids, not another authority.
So if a librarian goes to a conference and learns, "hey we need to remove these books from the lirbary because they are bigoted/racist/problematic" and they do so, that is not a book ban. But if parents say, "hey this book is not appropriate for our kids, this should not be in a school library", and they raise hell to get it removed, that is a book ban. The whole framing is dumb.