As a lawyer, here's the legality to the First Amendment. You're free to act as a journalist, but if you use someone else's platform, they're free to kick you off for any reason. If you start your own website, have your own server, you can do whatever the hell you want and have full 1A Constitutional Amendment protection.
Now, here's the part that have put Social Media Tech Stocks in legal jeopardy. There's actually lawsuits over Dallas Police Massacre and Pulse Massacre pending because of it. If a tech company provides a platform but doesn't unduly interfere on that platform, then the Tech Company enjoys immunity from any actions that occur in real life because of that speech.
Since Twitter and Facebook have been actively stamping out hate speech on the right while permitting ISIS to hold accounts and BLM to thrive, lawyers are arguing that their immunity has gone by the wayside because they have actually interfered in the organic flow of information.
These lawsuits have been permitted to go forward. I look for the trial judges to permit these suits to be heard, and I honestly expect a victory at trial, but these are more litmus test cases, and the way platforms are given latitude based off these cases will be ultimately decided by the Supreme Court.