If the helmets you see are on very young children, they may be plagiocephaly helmets, a therapeutic device used to correct head flattening. Our twins had to wear them. They had plagiocephaly, or head-flattening, caused by torticollis, which is a weakness of neck muscles caused by being too crowded in the womb. We felt very lucky that the (extremely effective!) helmets were available to us, as otherwise our poor kids would have grown up “kinda funny lookin’.”
TMI? :)
Last I checked, they don't use bicycle safety helmets for treatment - I've seen the plagiocelpaly helmets, these were just standard ride the tricycle helmets. I honestly figured they had just ridden bikes to the market (not unheard of in my area), but the family got into the SUV parked next to my car as I was leaving.