I guess another consideration is your personal style of
riding when considering a helmet. Someone such as
an oldie like me riding an e-trike at 15km/hour doesn't
really need a full-face helmet for example, but a young
bloke riding rough downhill mountain trails should be
wearing one.
I note from watching US biking videos that a lot of US
riders—particularly beachside cruisers—seldom wear
helmets, so I'm guessing that in most American states
there's no legal compulsion to wear one.
In my home state, Victorians are looking at a $227 fine
for not wearing a bike helmet. Ouch!