Originally Posted by GriddleCakes

Actually, the polystyrene is supposed to compress, not shatter (think crumple zones on a car). Helmets shatter when the stress load of the helmet has been exceeded and the helmet can no longer absorb any more energy, or provide any more protection. So it's not that they're designed to shatter, it's just that they're only designed to absorb a finite amount of impact energy, and beyond that they break.