Never mind why...
Crossing a road in a city in Vietnam, even on a green pedestrian light, is at first intimidating. Rules of the road do not appear to apply. Bikes run red lights, go up one way streets, or take to the sidewalk (pavement). Or all at the same time.
But in fact it is not very hard, or (apparently) very dangerous. Just go, and go slow and steady. Especially steady. As long as you keep going at the same slow pace, a path will magically open for you in even the densest oncoming wave of bikes, and traffic will flow smoothly around you. Takes a bit of confidence at first, but it works very well.
And whatever you do, do not run, abruptly stop, or change direction. Confuse one driver, and there maybe trouble.