Katamarino - what would they get sued for?
In aviation, the #1 cornerstone is that the
PIC (the "Captain" if you like
) is the final arbiter on the go/no-go decision.
He has sat 14 exams teaching him every piece of crap including the name of some wind blowing in Hong Kong. And then some more when doing the Type Rating. And he has flown through all kinds of sulphuric acid muck; any large 3rd world city is just fine for that.
He is perfectly capable of making a decision.
Volcanoes have been going off all over the place since long before aviation, so there is a big body of knowledge on this.
In this case, the airspace owners seem to have hung their coat on an ICAO recommendation that IFR in CAS should be pulled if there is volcanic ash there. But this isn't binding.
What the airspace owners (the NAAs) definitely can get sued for is if they ban aviation without a good reason supported by data. "Caution" is not a good reason. The economic loss is running at about 1M/hour. And you can sue for an economic loss, for sure.
IMHO, the NAA lawyers need their heads examined.
One way to proceed - until data is collected and analysed - would be a ban on night flight
in the relevant airspace. What you can't see can't do any harm. And with the high pressure around now, there isn't going to be much IMC; well not if your rate of climb is +5000fpm ...