Actually, Fennell, in Australia although accidents fell by soemthign like 30%, cycling itself fell by something like 50% -- in other words, the fewer people left cycling actually had more accidents, more often, than before the compulsary helmut laws were introduced.
The theory is that the remaining cyclists who wore helmets felt more protected by them and then took more chances; ergo, the net effect of having to wear helmets was to encourage people to cycle more recklessly. Makes sense -- I know that I am extremely much more careful if for some reason I go out with my helmet.
This has been a big reason why many cyclists still oppose mandatory helmet laws.
Also, it effectively punishes the cyclist when the actual hazard comes from big vehicles (remember the thread on here about why should women be advised not to go out alone when there's a rapist loose? Why should women have to modify their behavior?)
Why should cyclists have to modify their habits when nearly all of the road hazards are from vehicles/bad road design?
I wear a helmet myself, btw, am just setting out some of the arguments for why people think it's wrong cyclists be forced by law to wear them.
That and they can be uncomfortable or hot or even colder than a woolie hat. And the last thing people in this sedentary culture need is to be discouraged from exercise by not wanting to wear a "dorky" hat.
DH used to wear a bright yellow woolie hat; he felt that was safer than wearing a plastic hat.