It's definitely not a right if it disappears when it's inconvenient.
To offer more detail on how far the Brandenburg case went in protecting free speech rights: it ruled that seditious speech – including speech that constitutes an incitement to violence – is protected by the First Amendment to the United States Constitution as long as it does not reach a level "where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action”.
PS: And technically, afaik, it isn't a crime to call for the assassination of the president of the US. It's illegal to attempt it, and to take actions that support those attempting it. But if you just post on Facebook about it... you'll get a call from the Secret Service and a recommendation that you don't do that, but no charge.