During this confrontation two Police Officers arrived and asked the gentleman to leave, but he refused and was arrested for criminal trespass.
There's our answer: he wasn't arrested on a weapons offense. Whether or not it was legal to be there with a gun (and it was), once asked to leave, failure to do so constitutes criminal trespass.
The guy went out looking for trouble, and he got it. Looks like the lawsuit has already been filed
I'd lay money that he's going to try to make a 2nd Amendment issue about it, then scream bloody murder when the NRA fails to take up his cause.