There's actually quite a lot of concern from a number of different quarters about the extraordinary power that this private companies now have. They are the main forums for debate and discussion. And they are controlling what debates are allowed on these platforms. And that means they are able to exert considerable influence on whose voices win out in public policy. When they have such reach, and hence such power, just saying, well they are private companies, isn't really good enough. At the very least is blinding yourself to just how much power such companies are now holding. People are right to be concerned about this.
What's acceptable to debate and discuss has always been subject to the control of a few, and is entirely subjective depending on where you try to do it and where you live.
Previously the church or monarchy were the moral guardians of which views were acceptable, in the Europe anyway, then governments took on most of that roll.
The fact private entities now have a huge influence in public opinion is a double edged sword imo. It allows more people than ever to participate but as you say if they're controlling the narrative it will lead to issues.
At the end of the day someone, or some organisation, have to make a judgement call on what is and isn't acceptable to debate online/in public. It doesn't really matter if that's twitter themselves, the government or some third party group there will always be limits on what is acceptable to say and there will always be people who feel that lines are wrong wherever they are drawn.
But that's an entirely different discussion and as things stand today twitter hold the power to make the calls on what you can and can't say on Twitter.