Because trump didn't instruct his followers to do any such thing.
Here's the incident that the Trump-hater was exaggerating:
"I'd like to punch him in the face," Trump said, remarking that a man disrupting his rally was escorted out with a smile on his face. "He's smiling, having a good time."
Such heat-of-the-moment hyperbole did not constitute Trump "telling his supporters to punch people in the face", which would make whoever made such an assertion a liar.
You also should add that Trump didn’t think it was enough for police to take them out.