Why are helmets mandatory for cycling?

The issue of mandatory helmet laws for cycling has been growing in public awareness. This is possibly due to recent court cases and research, the proported lack of take up of the Melbourne Bike Share and the brand new Brisbane CityCycle program.

Proponents of repealing mandatory helmet laws generally focus their discussion on practical issues. They cite the lack of compelling scientific research regarding helmet efficacy in preventing serious brain injury, the deteriorating overall physical health of society and improving sustainability through lower impact on the environment. My question is should they be required to do that?

Supposedly Australia is a free, civilised country. We value our individual freedom. This freedom includes daily choices concerning how our own actions affect our own good, balancing our own personal risk against convenience, benefit and enjoyment. We make these choices whenever we drive our cars, fly in an aeroplane, jump out of an aeroplane, take up smoking, drink varying volumes of alcohol and go swimming.

By definition mandatory helmet laws restrict individual freedom. The cyclist is unable to make the choice of balancing personal risk against convenience, benefit and enjoyment. It does not matter what type of bike is being ridden, the speeds, riding surface, distance travelled and presence or absence of vehicles.

So my question is why isn't the government required to justify the presence of mandatory helmet laws, since they are exercising power over individual freedom, against some people's will? Such a justification would necessarily be in terms of preventing harm to others, otherwise it impinges on the individual freedom we as a society also claim to value. If no such reasonable justification is available then repealing the laws is the sensible and rational course of action to take.