When the government controls speech of private persons or entities engages in censorship especially when it compels speech. Private companies cannot engage in unconstitutional censorship. Go read the First again or maybe for the first time. See who it restricts. Private companies and individuals have the right to restrict speech. It’s very simple.
This forum is a great example. Mr. Robinson May toss any of us for any reason whatsoever at anytime. The government may not compel him to give a platform to those he finds objectionable. His restrictions though a form of censorship are not Constitutionally forbidden. If the government told Mr. Robinson that he had to carry all posts then it would be engaging in unconstitutional censorship.
The social media platforms have everyright to exclude any thing they wish.
Why do we have it? What need did the founders see?
What is the danger of not having it?