usually means
1. is usually exclusive 2. if not exclusive, then is the case in the majority of cases
The proposed case that "Book Bans" refer almost exclusively to school libraries is obviously false if you just take a look at a dictionary. It is evidently not "widely known to mean X" if common definitions do not explicitly state that.
All definitions state that it is an act of banning a book. But do not explicitly state that it is exclusive to some arbitrary bureaucratic level.
Either way, it is strange to just decide that a "Book Ban" must refer almost exclusively to a school.
And yes, this is semantics.