I agree with what you say FP. There are many reasons why a student could fall into this trap. Another maybe being that they did a straight in approach? If so, it's obviously easy to drag it in and get flat and without the familiarity and discipline of the circuit, miss the triggers for flap settings etc.
Another factor could maybe have been that the student was in fact acutely aware of the dangers of passing traffic and got distracted during the last moments of the approach when they saw the white car pass in front of them? Probably not helped by being a little pre-occupied by the pressure of having to make a decent landing for the missus filming it!
But irrespective of the many reasons and factors why it happened, the fact remains that they still made an absolute mess of the approach. Passing motorists obviously have no idea whether the pilot is a student or experienced. It would seem pretty obvious that most motorists were oblivious of the potential dangers of aircraft passing directly over-head anyway.
I think the over-riding issue is that there is a displaced threshold for a reason. There could be a double decker bus passing directly under the final approach and as long as you are doing a fair job of hitting the profile for the displaced threshold, relatively speaking you won't be anywhere near passing vehicles.
India 4 2 is obviously right - I cannot believe for one moment the student's instructor hadn't drummed into him about the displaced threshold and the obvious dangers of vehicles not stopping.
I have no idea of the intricacies of the US legal system and what the eventual outcome will be. But for the little it is worth, although I don't like to knock a student pilot, I don't place any blame on the car.
Fortunately, no one was seriously injured or worse, killed. As I for one, would not be posting. But in true pprune fashion, this video clip of an unfortunate incident, gives us all plenty to think about and comment on.
I would not be at all surprised if the last thing the student heard shortly before impact was a loud buzzing sound!!