Except that mandatory helmets discourages bicycling which causes disinvestment in safe infrastructure, and keeps drivers unfamiliar with cyclists. This makes cycling much more dangerous. Note again that the mass cycling cultures do not have mandatory helmets laws and are also much safer than Australia.
Also, it's weird that cycling is singles out for mandatory helmets. Fully half of all head injuries from individual transport happen on automobiles, yet nobody is suggesting mandatory helmets for car occupants. Even walking creates a larger number of head injury hospital visits. The arguments for mandatory bicycle helmets apply there too.
Ultimately, at a time that we need greater investment in mass cycling than ever, for individual safety and for the environment, mandatory helmets laws are counterproductive