It's all about that "styrofoam shell." (insert your favorite science names, like expanded polystyrene or whatever). The job of that stuff is to absorb most of the impact force in a fall, rather than it all being applied to your eggshell skull. The problem arises when that "ability to absorb and compress" has been utilized -- it's a one-time service for that foam. So, after you whack your skull on the ground inside your helmet, you have "done the compression," and the foam will not serve its mission in a future impact. So you replace the helmet after a fall and treat your skull to new, uncompressed foam. And it's not something you can necessarily inspect post-impact, so the rule is "you bang it and it's new helmet time." What the "it's OK" voters here are saying is: this drop most likely did not compromise the foam. Ride on, kiddie"

In response to Bald Paul ... I'd be full stop at "used helmet" regardless of a drop or not. Used helmet? That's just gross. Like used underwear.