Here in Canada, i can't wait for our government to make ski helmets mandatory to everyone on every mountain.
Why? Simply because if you get a concussion or other, the doctor treating you will be paid by MY taxes. So you better spend some money on protection to keep me from spending money on you.
As for the argument of creating a precedent (if we accept the helmet is mandatory, then it will be knee pads, then wrist guards...), it has never been and never will be a reason to stop a law. It's actually a very stupid way of thinking and everyone with minimal law studies will tell you so. It's one of the first myth that gets debunked in law classes. This is like people telling we should stop gay marriage becuz then, we will have no choice but to allow marrying a pig or a cow.
As for american friends, then it's completely different becuz if I understand correctly, no one is paying medical expenses for the concussion but the person who has it...