Yes, currently they can. The argument is that this is not desirable given how much sway these companies have over politics. For example, I have read that Donald

Yes, currently they can. The argument is that this is not desirable given how much sway these companies have over politics. For example, I have read that Donald Trump was elected as a result of Twitter; I wonder if he can win re-election when all of his support inevitably gets banned? It will be interesting to watch. If the public square has become the internet, it behooves the government to regulate it for political speech in the same way they do sex,race,religion, .etc.