There are costs and benefits to requiring helmets.

Requiring a helmet has apparently been shown to depress the use of bikes for every day activities. Going grocery shopping with a bike is less convenient if you have to juggle a helmet along with a liter of milk.

Less biking means roughly more pollution, more obesity, bigger roads, more car accide ts.

On the other side of this equation is the death and injury that helmets prevent. This is non-zero, but apparently in the Netherlands we have judged this marginal benefit of helmets to be less than the advantages of more cycling.

The calculation might be different in countries with inferior cycling infrastructure.