Just a guess, but I suspect a book gets banned because it's well known; it has content judged as immoral, unnecessarily sexual, etc.; and parents in an area suspect that teachers or librarians will try to convey the book to their children against the parents wishes. They thus take action through their school boards to draw a perfectly clear line.

BPL is not subject to any of those jurisdictions, so they're legally untouchable as they do the same work of trying to undermine those parents.