people that get suckered into buying CHMSL flasher/pulser; they're goaded into it by a salesman, assured that it's legal and "safer" and "X collisions are prevented a year" and even that they are specifically legal in some states
...except sort of not really, because under a principle of the US vehicle safety regulations known as federal preemption, if an aspect of motor vehicle design or equipment is federally regulated, states are not permitted to have or enforce a regulation of that same aspect, that differs from the federal regulation. Take headlight color: the federal standard says they have to be white, so a state can't require them to be yellow. Or turn signal color: the federal standard says red or amber, so states can't require only one of those colors. Or stop (brake) light operation: the federal standard says they all (including the CHMSL) have to be steady-burning, which means light of substantially unchanging intensity. So a state law allowing CHMSL flasher/blinker/"pulser" devices is null and void.
Caveats? Yeah, plenty: in practice, there's very little enforcement of this, except in adjudication of single-issue lawsuits and stuff like that. The feds almost never send a letter to a state legislature saying "Cut that shіt out!". The last time there was a big fight between the feds and the states was in the 1970s when certain states legalized European headlamps, which sent Joan Claybrook (head of NHTSA at that time) into a tizzy.
More generally, the way preemption tends to work is that states are prevented (more by industry pressure than by federal enforcement) from having requirements
stricter than the federal standard, but not much, if anything, prevents them from having requirements
more permissive than the federal standard. This is how states get away with "white to amber" for front turn signals, or New York State's eye-poppingly snotty/scornful language on the books about side marker lights
*, etc. So don't look for the feds to tap any of the few CHMSL-flasher-"legalizing" states on the shoulder about it...until someone connects the dots between these dumb gadgets and collisions, which I imagine will probably happen on or around
the first of Octember.
*On the other hand, New York began requiring rear defrosters effective with the 1974-model cars. This did not violate federal preemption, because no Federal Motor Vehicle Safety Standard requires rear defrosters. And for awhile, NY required quieter "city horns". This, too, didn't violate federal preemption because the applicable federal standard just calls for the horns to emit an "adequate warning" with no specification. This raises some interesting lighting questions: can a state require rear fog lamps? Or side turn signal repeaters? Legally, probably so! Industry would scream murder, and would try their best, probably successfully, to quash it, though.