There, the Court held that inflammatory speech--and even speech advocating violence by members of the Ku Klux Klan--is protected under the First Amendment, unless the speech "is directed to inciting or producing imminent lawless action and is likely to incite or produce such action"
Still supports my point: there are limits to free speech when it involves inciting violence (or other lawless action).
And there's still also libel laws which make lies illegal in some cases. Though I believe that's a civil rather than criminal issue.