Because if you legislate it, then each /time/ a person chooses /not/ to wear a helmet, it represents an active choice instead of a long-term habit without thinking.
Making the less-safe choice a costly option instead of a low-cost default is progress.
The only question remains is whether we want to achieve progress by legislation, or by some other tool like tweaking lift ticket prices to include a "No Helmet?" charge.
Personally, I'd /rather/ legislate than faff about with "Supplemental Brain Damage Waiver" fees.
You missed the point, there is already a high-compliance rate amongst those who participate regularly (frequency risk) and every hill I know of in Canada requires due to liability risks, that those who are the highest risk (young, beginners and infrequent), so they don't really have to make the choice anyways. It is already a from of self-regulation, albeit driven by liability and insurance, so why formal legislation??? Seems like there are better fish to fry.