should just change the laws to once your 18 its not a requirement to wear a helmet.
but if you do fall/crash/stack while not wearing the helmet then bad luck, no insurance/health insurance coverages for you.
aus is, im pretty sure the only country in the world where you do have to wear a helmet.
or is one of the very few in the world. however most riders ride with one because of the safety factor.
although personally i wont ride a bike without a helmet (unless its in my own driveway/or riding from the set up to the start gates at races, which is extremely slow speeds)
ive had too many crashes, to know why you should always wear them. most recently being on rocks in competition
but still, laws should be changed to 18+ not required to wear a helmet, but if damages do occur then stiff.
as for helmets not being safe.
it all depends on the purpose.
your skatelid aint going to protect you on a DH track. nor will a full face Bike helmet protect you on a Motorbike.
all helmets have to meet a specific standard and most are designed with specific speeds in mind.
as again an example. my MTB full face is designed for slower speeds then a motorbike full face.
however all helmets have to meet, from memory DOT and SNELL standards for certification.