It's not that the tshirt or the writing on the wall does anything by itself. The problem is that it encourages behaviour that other people thus encouraged might end up carrying out the act described in the message. To put it simply, it might result in white people being killed. This is why the "incitement to cause violence" is such a critical component.
"**** white people", which is hate speech, ought to be legally protected free speech. "Kill white people" should be proscribed, which it thankfully is.
Racist speech falls into the same category. It dehumanizes the victim and once that happens it is anybody's guess what might happen.