rBOB wrote:

It is perfectly
legitimate to ask: Why has society deemed it appropriate / necessary /
mandatory to wear a helmet while cycling but not while walking,
driving, running, descending staircases, etc, etc...)?

Possible answers:
We're moving much slower while walking
We have seatbelts and airbags for driving
We also moving slower runnning then biking
We have banisters to hold onto while descending staircases

The U.S. is a safety consious country when it comes to personal
activities. Right or wrong, helments are the safety item of choice for
any activity involving moving at a speed greater then running (roller
skating, skateboards, skiiing, etc..). It's an easy sell.

Rich