At least if private entities implement this kind of thing, those who wish can still discuss it someplace else.
Though pushing these things under the rug doesn't have the best reputation (see recent 8chan tied shooting).
Like - imagine if US was attacked and the attacker killed 60 million Americans. I can almost guarantee that no matter how strongly Americans believe in the 1st amendment, saying that it hasn't happened would be made illegal.
Why would scale change people's values compared to say 9/11? It's important to me that we're the deciders of truth for ourselves. I believe people fighting in those wars have fought for my right to believe what I choose and not what a government tells me.
Think about this in the context China, even today and I think you'll see why I feel this is an incredibly valuable right. Governments can lie too, the American one has many times throughout history and we should and do demand a right to question it.