As the court notes, what Trump said may have been taken by attendees as an invitation to inflict violence. Trump may even have hoped some would act on these urges even with the tossed-off addition of “don’t hurt ‘em.” But what he said was Constitutionally protected, no matter how unsavory or subjectively threatening the expelled protesters might have felt his statements were. For the court to step in and declare these words illegal would be a step too far.
I think this is the right ruling. It’s not illegal to be weak and ask others to silence critics for you. It simply means you are fragile and were probably raised as a sheltered rich kid in private schools.