> I don't see how helmets preclude cultural adoption.

Case in point. It seems completely common sense to me that helmets are a massive barrier to widespread adoption! Leaving aside the issues it introduces with bikeshare logistics, having an additional thing to keep track of and carry around and periodically replace (helmets can go bad!), here's something that I suspect you may not have considered: women! Whether it seems reasonable to you and me or not (and I'll be honest, it doesn't seem reasonable to me), the 2 reasons every woman I've ever asked gives for why they wouldn't want to use a bicycle to get around more are

1) sitting on the seat is awkward/uncomfortable when wearing a skirt or dress

2) helmets would ruin their hair

Requiring helmet wear makes cycling a nonstarter for a large chunk of 50% of the population (how I wish I had a citation for this! as far as I've been able to find, nobody has done any kind of survey that attempts to figure out if this is as widespread as I've observed personally). This alone makes me opposed to it, never mind the exaggerated claims of helmet efficacy

Try watching a video of people in Copenhagen riding bikes. Women wear dresses and skirts there all the time while cycling. Same goes in Japan; women commonly wear dresses/skirts there and ride bikes.

And, yes, people are sometimes forced to take other steps that are primarily about their individual safety, such as wearing seat belts. I mostly disagree with those laws as well.