Why?
Because wearing a helmet while walking round Tesco doing your shopping and you will look something like this:
Wear a helmet while setting a speed record on your push bike and you will look more like this:
Of-course there's nothing legally or even morally wrong with wearing a helmet while walking around Tesco's any more than it is to make babies while wearing a bullet proof vest, but you will look a bit

On the contrary, I do not think not wearing a cycle helmet should affect compo payments if some arse bandit rams you up the chuff while cycling.