Proportionality is everything. "White people have no rhythm" is quite a bit less extreme than "I'm going to urge folks to tie nooses around the necks of white people and drag them behind pickup trucks."
You are absolutely correct- there is a huge difference....to you. I don't like it. I find it very offensive and degrading. The black kid could have shown he was better than the moron by ignoring him. I don't think the drawing on his t-shirt urged people to get married. Do you? Or, did it urge people to go to church? The message you are hung up on (pun unintentional. really) is the lynching. As abhorrent as the display was, it doesn't justify the assault.
(I wish there was a way to get inflection in text)