I think this kind of behavior is considered a ban because it doesn't take a formal government decree to effectively preclude someone from being able to access u

I think this kind of behavior is considered a ban because it doesn't take a formal government decree to effectively preclude someone from being able to access undesirable literature. If your school/community library is your main source of literature, then if you can't get it there, you can't get it anywhere. This argument probably held truer a decade ago, but I'd wager that it's still disproportionately true in less wealthy/technical communities.