Helmets should be mandatory because of the high risk of head injuries and the ensuing societal cost. When someone is in an accident and has a severe head injury, it doesn't just change their lives, it changes the lives of the people they love. It also costs taxpayers millions of dollars to care for these people. Wear a helmet!!
Skiing and snowboarding can be extremely dangerous. Going downhill at fast speeds, sometimes losing control, and with obstacles along the way make for a risky sport. Not forcing people to wear helmets would be irresponsible. Even with helmets, professionals have died because of accidents. Sarah Burke, a gold medalist in the X-Games had a severe accident and passed away due to the injuries, even with a helmet. Now how could we take off helmets from those who are not even professional?
I agree that people should wear helmets, and as a snowboard instructor I always recommend them to anyone BUT this is very different to enforcing the wearing of a helmet. People should be able to have the freedom of choice, too many rules can often cause as many problems as they solve. I wear a helmet when I feel the risk is high enough, we should focus more on ensuring riders and skiers are safe both on and off piste instead of trying to bombproof ourselves so theres less of a need. This means a better, more concise and universal understanding of what safe practice each, instead of each country having its own rules. I won't go as far to say we should have exams or anything else (like driving for example), because people should be free to buy some gear and try it out themselves, but when they do decide to get lessons, make sure the lessons are to a consistently high standard, focussing on safe practise as well as technique. This means slope users are able to identify when a helmet should be worn but also how to conduct themselves safely on piste.
They do protect you but it is an expense that some don't want to pay for. If you are staying in control and not booting off 40 ft jumps you should be able to choose weather or not you get to wear a helmet. They make the helmets over proctected and they are very large and uncomfortable.