Not wrong. The clerk was a jerk. He should have been fired. That being said the pro Trump guy was a troll. Not much different then some gay guy that trolls Christian flower shops and bakeries trying to get something juicy for YouTube.
That's just your opinion. You are creating facts not in evidence. According to the facts as presented, he was just a shopper looking to buy a product that happened to be wearing a Trump shirt. He was attacked for it. Are you saying that if I go shopping in a Trump shirt that I'm trolling for trouble. If so, then go screw yourself. The last I looked, I still live in America and am entitled to wear apparel that supports the President of the US without being attacked for it.