I'm thinking in a city environment. Motorcyclists ride in much more varied environments at much higher speeds with the other vehicles at higher speeds. Motorcycling is very dangerous, period.
But in city riding, motorcycles, cars, and bicycles are effectively bound to the same speed limits, such as between 0-45mph. Obviously, bicycles will be slower at the top end, but for the most part in congested city riding, all three vehicles will be within some range of each other.
In my experience and observations, the common motorcyclist will still be far better equipped in safety equipment, including helmets, gloves, jackets, boots, etc. despite having superior protection from the motorcycle itself compared to a bicycle, breaks, and acceleration. And also in my experience, bicyclists are much more willing to break traffic laws in city environments.
Maybe the comparison isn't appropriate or too heavily biased with my own safety approaches and experience, but in my experience with serious motorcyclists, anything less than a full-face helmet is viewed as complete folly. The highest percentage of hit in a motorcycle crash is in the front. (There is a famous safety graphic that shows the percentages of where on the helmet a crash occurs.)
If I decided to start riding bicycles again, on the street or elsewhere, a helmet will absolutely be used, and I would even consider going further than the typical bicycle helmet. My real point is that I find it extremely strange that some bicyclists would choose simply not to ride at all just because a helmet is required.
> and more often as the result of their own actions
That is not a blanket truth.