When you've been around awhile, you see lots of curious things. With respect to MC helmets, I recall:
1. I was stationed at Ft. Carson in the sixties. Colorado didn't have a helmet (or anything) law for riding motorcycles at the time. But the Army did. One of the guys in my Company had a Honda Dream for commuting to off-base housing (he was married). The Army was fine with him using a "steel pot" helmet for riding. These were crappy things with feeble latch/chin straps that would pop off your head with the least provocation. I hated them for any use, never mind this.
2. Later on Colorado had an early helmet law with no stipulation as to what constitutes a "helmet". A construction-worker helmet would get you by.
3. Later in Colorado, they repealed the helmet law for those over 18. But you could be dunned for not wearing eye protection.
4. In New Jersey in the '70's you had to take your bike through those safety inspection stations. They'd check, among other things, your headlight alignment (a questionable exercise with the headlight mounted on the forks/handlebars) and inspect your helmet for make/model - in those days the DOT sticker might not be on a helmet you'd buy. (I actually did fail once for headlight alignment. I went to the back of the line and through again and passed.)
5. Once, while riding the Clinton-Mukilteo Ferry (Washington State), I overheard a fellow biker (A Harley Rider) regaling a non-biker passenger with the dangers of riding with
a helmet. Among these was an assertion that wearing one in extremely hot weather, like in Arizona, your head would overheat and your brain would sort of - explode.
The differences in State laws I can see as an annoyance for those who live in States that allow "au natureil" heads while riding who have to carry a helmet when traveling into/through States that do not. Fortunately for them they can make the political statement of wearing those tiny cranium buckets when necessary.