Who said he was "joking"?
It was making light of lynching. Treating it as funny. That could be viewed as "advocacy," although it isn't direct advocacy.
But what about "Freepers should be shot"? (Head of Hillary Now)
Or "Someone should kill President Bush"? (Alec Baldwin)
Or "Someone should kill Henry Hyde"? (Some Democrat whose name I forget").
Or what about the pictures on this thread, advocating fragging military officers? You can bet that if a patriotic American took a swing at any of those jerks, he would bear the full brunt of the law, and the jerks wearing the inflammatory t-shirts would have gotten no punishment at all.
There's a double standard here. Offensive speech should be met only by more speech, not violence.
Do you think someone could have gotten away with physically assaulting any one of those people? They were directly advocating violence.