Second hand information from a fellow who used to post on rec.autos.driving who was heavily involved in this sort of thing. I don't have access to where his numbers came from, but he was a respected resource there.
It means that if you have enough for one pie, do you want one that might kill you or one that will definitely not kill you?
Great, then make them optional. I'm not telling you not to use them, I'm telling you that I don't feel comfortable with them. How can you justify your belief that airbags are great to someone who has lost someone to them in a low speed collision? "Sorry about your luck"?