Actually, That's incorrect -- the current standard for federal regulation of free speech, set by the courts in Brandenberg vs. Ohio, states, "speech must not only be advocacy to incite imminent lawless action, but also there must be a likelihood that such action will occur."
Therefore, the mere advocacy of violent overthrow (a la the communist party, which advocates a violent revolution against the capitalist government) is not illegal -- it is the advocacy with the intent to incite imminent lawless action. Nor is "Damnit, that guy should be shot for being so stupid" unprotected -- because there isn't a likelihood that a reasonable person will actually follow the instructions.