The problem is with situations like where our President tweets gifs promoting violence against people and news organizations, or attempts to instigate nuclear war via Tweet. Trump is obviously a gold mine for Twitter right now from both a growth and ad revenue standpoint (not even getting into the bot side of things), so they have a very vested interest in looking the other way.
There's also a lot of dangerous precedent that could be set if they were to block the POTUS.
So do we just say "no violent speech unless you've somehow managed to get elected"?