i disagree entirely.
smack your melon hard, and the foam as rightly said cushions the impact of your skull against the helmet's hard outer shell.
few things if anything are perfectly elastic and so the capacity for this absorption is reduced following a single hit.
of course it is up to you to quatify the severity of an impact, and thus the compression of the foam, and thus the need to replace yer lid.
my train of thought is just personal opinion and perhaps biased from riding motorbikes... but if i were to whack my head, i would buy a new helmet. and a good enuff whack to the head would not necessitate a concusion or anything remotely as severe.
but as said, this is just my opinion, so take it for what it is worth.
Just coz you don't understand it
Doesn't mean it makes no sense!