I think you might be falling into the trap of thinking of a government as a monolithic entity, that has coherent goals and decision-making ability. That is, of course, false, and the actual case is that the government is just a group of people. A group that can be infiltrated.
So, the obvious answer is the same as why CCTV (China's state communication organ) would show
V for Vendetta: Someone within the power structure is friendly to libertarian ideas, and have placed themselves where they can do some good by authorizing seditious programming.
I generally like reading your post, it's interesting that as an anarchist you know more than a lot of statists about how real-world politics work.
As for China, it should also be pointed out that, is there is not a collective self-consciousness for a movie like "V for Vendetta" to provoke repercussions, to the extent that perhaps even the censors failed to get the movie's point. Last time I checked, in Amazon China "1984" was given away on a "buy one get one" basis.