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.
I think there is a lot of research into the subject and that is why we err on the side of caution; volcanic dust is far more dangerous than we used to assume. Its not just that an aircraft may lose all its engines in flight, but that all aircraft who fly through this **** may have engine damage.
Would you be happy to fly on a aircraft that has pumped tonnes of acidic and abrasive dust through its engines (and with a certain amount nicely coating every component) without being stripped down and inspected? I would not.
You cannot know what the effect will be on some engines; is it really worth flying through this and then requiring they are immediately overhauled to inspect/repair the damage - on thousands of aircraft, as an emergency procedure. How many billions would that cost and how many months disruption would it cause?
Edit:
KLM Flight 867 in 1989, Boeing 747-400 less than 6 months old -
"In this case the ash caused more than US$80 million in damage to the aircraft, but no lives were lost and no one was injured."