You are assuming that a simple free market of ideas actually works. As it turns out it really doesn't because people are stupid and when you let people self select whom to attend to stupid people choose other stupid people to attend to and soon mistake constant repetition of falsehoods for truth.

These spreading piles of human bile attract people who are peripherally socially connected on the social graph some of which grow the pile o poison until you reach a critical mass of poison and your society derails.

You advance the fallacy that if we censor anything we must censor everything that powerful parties desire. This isn't so. Facebook has actually been very reluctant to censor anything at all and even now and is only now choosing to censor things that are harmful uncontroversial falsehoods.

In the case of the holocaust denial they are hateful lies that have festered for decades. Nothing in particular forces us to slide down the slope and start covering for the powerful.