Nobody in Amsterdam wears a helmet while biking. The one exception might be sports cyclist who ride at high speeds. They usually won't show up in the inner city tough.

Biking in Amsterdam is pretty safe. But this is mostly because of the infrastructure, and more importantly because dutch drivers are used to cyclers on the roads. Most bike accidents in Amsterdam are caused by foreign drivers that are not used to looking out for bikes. I imagine this the number one reason biking is less safe in American cities.