I am currently on a small Greek island where no one except the occasional tourist wears a helmet and the ones that they are issued with, on request, appear to be made of eggshells.
Every time there is a new chief of police the helmet law is enforced until he realises that riding through narrow streets in 38 degrees whilst avoiding unlicenced tourists who are trying to see how many fit on a quad bike requires all your senses and wearing a helmet is not conducive to reducing accidents.
In actual fact the majority of accidents, some fatal, are from idiots showing off after leaving nightclubs or from poor girls coming off whilst wearing bikinis and suffering severe grazes.
Off topic, but a warning to any yoofs visiting Greece. If an alcoholic drink is too cheap to be true it will be Methanol and far more dangerous than not wearing a crash helmet.