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.