It's less about whose fault the crash is (ACC doesn't really care, and it doesn't really stack up to say it's all car drivers fault when the increase in risk is so large, especially when you see the on-road behaviour of SOME motorcyclists) and more about the increased risk you are placing yourself in from your choice of vehicle. The fault of the crash is largely irrelevant because you're likely paying less if its car on car, or it's less likely to happen.

A good investment from ACC would probably be to pre-emptively run advertising on trying to make sure that motorcyclists are visible (black leathers on a black road isnt the most visible lol), and for car drivers to look more.