On: Terrorists Mock Bids to End Use of Social Media

Calling for mass murder isn't a valid case of free speech.

And even if it were, Twitter can ban accounts for any reason.

"Calling for mass murder isn't a valid case of free speech." To be pedantic, yes it is, free speech isn't conditional on the message.

Your second statement is true, 1st amendment applies only to the government ability to restrict speech.

I should have said "isn't a valid case of legal free speech in the United States".