This is the important point.
If a book is already published then banning it is great for sales. But if it is aborted before seeing the light of day then what?
I suppose that is where self-publishing comes in as was discussed previously.
The Communists tried to strictly control book publication and this lead to Samizdat – self-published, underground press.
Anyway, I am not surprised that both sides try to stop the proliferation of opposing ideas even though it seldom if ever works. The real solution is to provide a better idea.