You have a good point.
However, (and I don’t have the moments necessary to source this) existing law protects publications/forums/services that don’t exercise editorial overview (and I’d bet some gentle culling is supported by case law).
On the other hand, once a publication clearly exercise control over what’s permitted, it’s then liable for what is published.
By taking these steps, Twitter is making itself liable for all the gross things that it has chosen not to censor. Investigations showing celebrities paying for fake followers, and the ‘infamous’ 13 Russian bot-masters, will not save them from this liability.
Of course, someone actually has to take them to court on it, or the liability is nothing.
Not only that, should Twitter and the like be selected to serve a “public function” such as early warnings for public benefit as related to health and welfare, mass notifications of harmful public-related events/incidences, etc., they may be forced to “behave” according to public laws in those instances. So the tail may come to wag the dog. :-)
Some damage to Twitter might be accomplished through the legal system, but I am in favor of damaging it and other communications systems controlled by the left, with whatever tools may be available.
In my mind, the point is to destroy their ability to censor, and I will likely support any reasonable plan for doing so.