We live in an imperfect random world that is inherently not perfectly mesurable - zero tolerance policies are inherently dumb. Find any VA ? - have to close airspace. That's the easy part.
Now how do you reopen it ? How do you prove there is zero VA ? How many test flights over how many days (remembering you've just grounded your test aircraft for repainting, after the eruption started...) have to come up clean ? What if you didn't cover the whole area and missed some ash, what if your instruments aren't sensitive enough, how do you prove there is zero VA so you can re-open airspace ?
Since our world is so imperfect and random, we sometimes have to resort to the best information available when making decisions, even knowing that information is imperfect. The best information the government authorities had were the cloud models, which predicted significant ash over a wide area. When the lives of so many people and the reliability of so many engines is at stake, there is no sane choice but to be conservative IF, as is proclaimed by all the regulators AND the airlines, "Safety is Paramount" is indeed the truth. Personally, I applaud those regulators for deciding it is time to do more than pay lip service to safety concerns that happen to have significant cost and corporate and personal inconvenience.
How many of those airlines would have accepted strict liability for any damage and/or injury caused by ash ingestion? I suspect not one.