Control. Trump is not controlled by Twitter. He is controlled by the government just like the rest of the US. A school, they control the children, and they have a limited control over the child's free speech. Your job, possibly a private institution, has control over you. They can censor you with the power they have.

We need the 1A because we don't get to choose our government, we do get to choose our internet messaging platforms and they choose to work with us.