The BMC (British Mountaineering Council) does offer specialist insurance for mountain sports. I don't see why it should be mandatory though. The reality is that the vast majority of people that go into the mountains (or the bush) appreciate the risks and take due precautions. Most 'normal' people who don't know how to look after themselves have enough common sense to know their limits and don't get into trouble either.
The minority of people that get into trouble are generally those who lack a bit of common sense. Darwins theory at work again....
The very very small minority of experienced people who get into trouble knew and prepared for the risks as best they could, but sometimes nature just outsmarts you.
The day I accept anyones suggestion that mandatory adventure sports insurance is necessary to enable the emergency services to 'reclaim the costs' is the day after all the drunks, druggies, smokers, obese people, irresponsible drivers etc etc take out extra insurance that enables the emergency services to reclaim the (much greater) costs incurred from them.
Will be interesting to see what happens in the UK after the SAR choppers are privitised though.... http://www.defensenews.com/story.php?i=3984057