Well, that's their problem. Also take a look at how 3 billion people ride their motorcycles and mopeds (often 3-5 at a time) in e.g. China, India, Thailand, Cambodia, Vietnam, etc. and the complains against ebikes are absolutely nullified...
If you look at the road mortality statistics for those countries you'll see that safety equipment and regulations are absolutely necessary if you want to avoid lots of pain and suffering, and externalities. It's just those places simply can't afford them and deal with it in ways more advanced economies aren't prepared to accept.
I don;t live in Cambodia. When people here get hurt in my neighborhood they block traffic, are taken to hospital and are treated by teams of government-paid professionals. Then if they have severe head injuries that medical treatment may continue for many decades. We insist on helmets, as we insist on things like brakes and seatbelts, because we know that individual injuries quickly become everyone's problem.
People are actually aware of motorbikes, since it's the primary method of transport. Also, traffic moves a lot slower, you're usually riding at only 20-30 km/h in town in Vietnam. I felt much safer riding a motorbike in Vietnam than in New Zealand.
Upon saying that, the road injury and fatality statistics for the countries you mentioned are shocking. Vietnam is 2.5x higher than the USA for road fatalities per 100,000 per year, and the USA itself is shockingly high for a developed country, they're double Australia's rate.
Just 2.5x doesn't sound shocking to me -- and probably the most of the difference is not even caused by driving patterns.
E.g. the road conditions in Vietnam vs the US, and the vehicle conditions there (breaks, mechanical faults, age, etc.) compared to the US...
You could say the same of seatbelts, yet their use is enforced by law because automobile fatalaties were a major public health problem. Society has an interest in protecting those who don't protect themselves.
Was it because of that, or merely because the government just likes to overreach and regulate?