They're flouting the law, good manners, and they have things that explode. Consider it common sense to not necessarily want to approach people directly. 

We were sitting near my mother's house watching various (mostly legal) ground fireworks up and down the street once, where the neighbors started shooting up illegal fireworks.

One of them went off sideways and came within a foot or two of me and my then-four year old  as it whizzed off horizontally a foot or two off the ground. It took me months to stop being jumpy--my immediate reflex was to cover my child with my body as the thing went past. 

These things are illegal for a reason. Between them and the ones that just shriek for the sake of shrieking... I can see why people get upset.

I suspect if they were restricted to NOT the ear-damaging loud ones and peoples actually followed the law on the launchers, there would be a lot fewer complaints, and still plenty of pretty explosions.