It's a pinnacle of ignorance

I've seen it happen firsthand a couple of times. In both cases it was done after the cyclists stayed two abreast instead of singling up to let traffic pass.

Cyclists, especially in a group should stay two or three abreast as it significantly reduces the passing length for overtaking cars. The only reason people complain about cyclists two abreast is that they want to overtake when it's not really safe to do so.

Does the law say anything about having to get into single file?

(In any case, the law doesn't provide for this case of vigilantism. And two wrong wouldn't make a right.)

Of course not. The law typically requires slow traffic to pull over when safe and allow faster traffic to pass when there are more than a few cars backed up behind, but that's a bit different. But naturally the lack of legal standing doesn't stop drivers who think they have the right to pass all bicyclists as soon as possible.

Is it even legal in USA? In EU, that would make your card road illegal instantly.

A police officer once told me his favorite traffic ticket to write was "excessive visible emission".

I've noticed a difference in the approach to enforcement of many minor regulations between the US and Germany, where I've spend a considerable amount of time. In the US, it's common for things like vehicle emissions and safety to be subject to many regulations, but only checked if somebody complains or the police happen to notice. In Germany, cars are regularly subjected to fairly strict inspections.

So people in the US do a lot of things with their cars that aren't technically legal. For the most part, this doesn't seem to be a major safety issue. In an analysis by the Department of Transportation[0], 4.9% of crashes were found to involve a tire/wheel deficiency and 1.9% involved some other vehicle deficiency. Tire pressure monitoring recently became mandatory on new cars sold in the US, which should have a significant impact on the most common maintenance issue. It's likely that a comprehensive vehicle inspection program would not have a particularly good cost/benefit ratio relative to, for example improving traffic flow and signaling at intersections. 36.2% of crashes involved intersections.

[0] https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/...

It's not legal. Even in states without emissions controls, you're still violating federal EPA regulations. But the feds have much bigger fish to fry and don't care to enforce stuff against individual car owners.

Depends on the state. Some states do emissions testing that fails such people, and officers are on the lookout for vehicles that are visibly violating the standard. Other states are indifferent as long as you aren't assaulting others with it.

Obviously, seeing as how they're doing it for attention, it's very unlikely that someone would roll coal with the intention of not blasting someone with soot.

No, it's not legal, but AFAICT it's usually activated with a switch, not always-on, so the cops would have to actually catch them in the act. Also, this kind of thing is surely almost always done in areas where there's no emissions testing (such as the South), so it's just not that easy to catch violators.