If as a society we allow companies, that are equal to (if not more powerful than) the government we elect, to pick and choose what parts of public discourse are acceptable or not, we have no one but ourselves to blame when this inevitably goes pear-shaped.
Like many other things in the past, what is legal right now may not be ethically and morally correct in the future.
You are correct that society as we know it does not cease to function immediately after the giants like amazon, facebook, twitter, ebay, etc. take a specific action like banning a book or banning a user. However, the longer term implications are far more concerning and the time to start contemplating these issues was yesterday (or any other time in the last few years). The next best time is now.