What is wrong with stopping someone from committing an assault? If a person is throwing an object to harm another person, there is nothing wrong with stopping it. Or if someone wants to defend themselves with a gun they need to be shot first before they can respond.
If you're God or a mind-reader....nothing. But acting pre-emptively on what it 'looks like' somebody is going to do is illegal. At least...at the moment. What Trump is advocating here is union thug tactics. You know....like Obama's tactics (if they bring a knife, bring a gun). Not presidential material, IMO.