I don't really care if someone I don't know and have no connections with chooses to ski without a helmet. And I don't certainly don't care if it's not made compulsory - too much nanny state as it is. But if your brains are smashed to a bloody pulp (or you get a fatal concussion) on the piste, and a helmet would have saved you, a lot of people will be affected.

The people you were skiing with
Anyone around who witnesses the accident
The person(s) you collided with/who collided with you
The rescue services
Your friends
Your colleagues
Your family

Possibly the only person positively affected will be your undertaker and coffin maker.

Or have I misunderstood what you mean by "how can you argue with something someone chooses to do when it cannot possibly affect you?"