That doesn't explain why they've banned Justin Bieber, Winnie the Pooh or celebrity gossip. If anything, those would distract conversation away from the party. They're also trying to be a moral police.
Personally, I think not all censorship is in some way aimed at free speech, that's just the most obvious parts.
Anything the party leadership disagrees with can potentially be censored. That includes criticism and ridicule of the party, but also pornography or memes they don't like.