Moderation where A dictates what B does/n't read is against freedom of speech. Self moderation whereby every B mods down unwanted material is compatible with freedom of speech, as is a mechanism whereby B indicates "I agree with C and want not to see what C doesn't want to see."

B and C are simply exercising their freedom from speech, which is as important as freedom of.

Of course, this has its own problems: like online fora devolving into cliques of people who think the same way.

People should be exposed to trolls and critically evaluate and re-evaluate their beliefs.