I guess it's pretty easy to make a wrong decision for someone who is not experienced. It's a national park and there are roads and ranger stations and such, so it doesn't feel very wild or remote. There were signs recommending 4wd, but the government likes to treat people like babies so many people don't take these signs seriously. When their car got stuck I imagine it was probably very hot inside the car. Maybe they figured that if it's hot anyway, might as well start walking. Walking actually doesn't seem that difficult at first. When I lived further south I was able to easily walk 1 hour in that weather. Of course 1 hour on pavement is not the same as 6 hours on sand, but by the time they realized it they might have been too far from the car.

Incidentally, what are some strategies to stay cool in a broken down car in a desert? Cover the car with tarp or mylar, open the doors, have ice in portable freezer. Any other ideas?