I suspect this is motivated by performance concerns. The overhead of the centralized filtering they do is massive, and a friend living in China tells me that it hurts net responsiveness badly. Since the Chinese government already seems to have decided a while ago that it's only interested in preventing casual access to subversive material and doesn't care about anyone determined enough to bother connecting through a proxy server, it makes sense to decentralize the load by moving the filtering logic onto the client.
If I'm right, though, this presents an enticing opportunity for some vigilante freedom fighting: write a worm that disables the filtering software.
