The article didn’t say he “fired” the “weapon”, they described how it would operate. If he had “fired” it they would have probably filed some kind of charges immediatly or have shot him. I think they’re researching to see if they can charge him with anything.
Another article says the police had numerous calls from people saying that someone was firing the gun at people, while the vehicle was being driven down the street.
Even on Independence day, that might be said to be disturbing the peace.
It appears that this is not that clear cut, because the only offence mentioned is a fireworks offence.
http://www.candgnews.com/news/july-4-panic-caused-antique-military-vehicle