Twitter added fact checks to two of President Trump's tweets regarding mail-in ballots. A small blue link at the bottom of the tweet encourages users to "Get the facts about mail-in ballots."
This is not a violation of free speech. For one, the president's tweets are still visible. Additionally, free speech is a concept passed from government to citizen, not the other way around. As a private company, Twitter is not beholden to the constitutional concept of free speech and therefore cannot violate it. Twitter is free to regulate its private platform in any way. Free speech, just like any constitutional concept, only applies to the government.
Furthermore, Twitter's action of sticking a fact check on Trump's tweet is itself an act protected by the First Amendment.
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