Imminent lawless action would be an impossible standard, and if that's your standard, then accepting harsh KKK political speech should be within your realm. Maybe they step out of line sometimes, but they know how to play the Westboro Baptist game. There's so much hate speech that can't be obviously said to incite violence.
Also, at the heart of banning bad speech is the fear that your fellow countrymen are too weak and stupid to deal with political, racial, sexual, or just scientific thought in general, and you fear the democratic reigns in their hands.