When I was in Iran, the government there blocked Twitter, effectively deciding for an entire nation what they cannot read. In America, Twitter itself purges users, effectively deciding for an entire nation what they cannot read. It matters little whose hand is on the switch: government or corporate, the end result is the same. This is the America I always feared I'd see.
Speech in America is an inalienable right, and runs as deep into our free society as any idea can. Thomas Jefferson wrote that it flowed directly from his idea of a Creator, which we understand today as less that free speech is heaven-sent so much as that it is something that exists above government. And so the argument that the First Amendment applies only to the government and not to private platforms like Twitter is both true and irrelevant—and the latter is more important.