Of course, it's also pretty clear that Branson is angling for a handout here, not really deeply interested in science or public policy. He has a pretty big self-interest in convincing people that the cause of the shutdown was government overreaction, in which case the government should compensate the airlines; rather than having people believe that the shutdown was a necessary reaction to the volcanic eruption.
It is an interesting point though. European regulations, intended to stop airlines leaving passengers in the lurch because they over-booked a flight, have made airlines the insurers of last resort for people stranded due to natural disaster. The airlines have incurred a lot of costs, not just on lost business but also on having to pay for accommodation and food for their passengers while they've been stranded. They're also liable for reasonable alternative means of transport. There's a nice summary on the BBC.
While this is nice for passengers, the airlines themselves have no-one to lean on. As this is an act of God, the insurance won't pay out. The government closed the airspace, and is the normal insurer of last resort for natural disasters (see Chile, Haiti etc.). While the summary rags on "greedy airlines", this is not the fault of an airline having mis-judged its margins but an unprecedented restriction on normal business. I think that there is a good case, in moral terms, for at least having the EU refund the costs brought on by passenger rights legislation.
I'm not crticising the government for closing the airspace either, to clarify, I have nowhere near enough knowledge on volcanos or jet engines to form an opinion. I just think that we should note that the airlines have been forced into an uncomfortable, and I think unjust, position.