The fact of the matter is that there is an uncertain amount of risk associated with flying through a volcanic dust cloud. Probably most aircraft would be unaffected, but there is a chance that something would happen in a small number of scenarios.
The two or three well known instances have survived by good airmanship and luck. They could very easily have gone the other way.
So who is willing to take the chance? Is it 1/10,000, is it 1/100,000? What is an "acceptable" level of risk, and who determines that.
Who would take the responsibility if the 1/100,000 chance went wrong?
In this age of litigation at the drop of a hat the legal liability would be horrendous.
No doubt this huge disruption will spur new research into the effects of volcanic dust and the forecasting of danger zones, but in the meantime we have to err on the side of caution.