I think you missed my point. If there was a call for violence like in your hypothetical scenario, then the tweet should be removed and legal action taken. It is not covered under any free speech protections.
I think it's rather you who missed my point. Once it's out there, it's out there. They may take it down but it'll be too late.
Then you might as well not let anyone talk about anything, anywhere. What you describe sounds a lot like the movie Minority Report.