Consider that the country with likely the least rate of helmet use, The Netherlands, has the lowest rate of bicycle fatalities (7.3 / Billion km) and is the only country in the world where it is safer to ride 1 km on a bicycle than 1 km in a car.
Two countries with the highest helmet use in the world, U.S. and Australia, also have the highest rates of bicycle fatalities (76 / billion km and 52 / billion km respectively)
Why is that?