I was going to say the nrma would likely tell you to contact the RTA equivalent and the police, then I saw "50 states"...... You yanks tend to have a lot less regulation than us, whether it is lack of necessary indicators or lack of a need to put number plates at each end of your car.
However, if it isn't the state you are in (easily checked), won't that make it a criminal travelling interstate to commit a crime, and that be a federal matter? Perhaps check with your own state regulator, then go federal?
I've driven a car through and within about half of the mainland states so far, the number of cars I saw that had flashing tail lights instead of any indicators (we call them blinkers, the orange lights on the corners of cars) was surprising. All of them weren't registered and were illegally on the road? Seems unlikely. Yet you say they were illegal? Fascinating. It didn't look that way at all.
And yeah, plates were indeed only on the back, but that is a different weirdness, especially that they had other things on the front. Odd "vanity" plates on the front which did not match their back plates; Some humorous, others more along the lines of scatalogical. Seemed a good way to make sure people wouldn't know who committed a crime if they only got a shot of the front of the car. (And don't get me started on lack of speed regulation in the US....)
(I will be using my made-up terms, which may be confusing.) The USA is an "after the fact" cleanup place. I have heard Germany is a "before the fact". Which may explain a lot.