My father is a long-time firefighter in a major city and this comment reads like you have no idea what firefighters actually do if you think they "do nothing compared to paramedics and the ambulance crew". They're two sides of the same coin and both of them are vital to rescue operations, but firefighters aren't just some dudes who roll up and pour a bucket of water on a campfire and drive away. They're EMTs, too, and often participate in casualty care when paramedics are busy or otherwise not on scene. Do nothing, my ass.
In my experience, it's mostly a vet/military thing. Rednecks fly the actual confederate battle flag. It could be cop, but they typically fly the thin blue line flag. Subdued/monochrome flags are pretty popular accesories in military and military adjacent communities. If you check out "Grunt Style" you'll probably see dozens. And enlisted (grunts in particular) tend to skew less educated and are more entrenched in toxic ideals of masculinity, so it's no surprise to see them acting like fools on the road.