Given American policing culture, I really don't buy that the police can't show up and start arresting you for public endangerment. They may not want to, but that is different
He said the trick is to invite a couple of cops.
If you have a cop or two at the party, and, often, a volly firefighter running the sparks, it seems to be OK.
A few years ago, one of our neighbors was running one, and a mortar fired into his second story. It was a 5-alarm fire.
They mind because they are completely outnumbered. Policing only works if you can point out a small minority and say “these people are causing harm to the majority of us”. With something like fireworks (or cannabis protests in many places) the police are severely outnumbered and they don’t have enough room in their jails anyway.
The "market of stubbornness" is about one deep at any given moment.