I am utterly confused. Either it is safe or unsafe and there is a (not necessarily constant) boundary between the two.
No, its not black or white, its a risk assessment.
Statistics. Compare it to smoking - will a cigarette kill you? Maybe not but it will increase your chances of dying. Exposure to different levels of ash will increase the probability of damage.
This is obviously what has been decided on - a risk that airlines will take on for themselves and decide if its worth it.
Seems perfectly fair..