36050100 has it exactly right. European law says that the risk lies with the airlines. The airlines are aware of this and (doubtless) can insure against it. If an airline chooses not to, tough.
Many individual travel policies will not include cover for this type of event; and in this case, IMHO, an appropriate allocation of risk (in circumstances where it is hard for an individual to insure against this kind of event) is that the risk lies with the airline.
MOL seems to be arguing that the cause of the delay was not actually the eruption but the government's (alleged) overreaction to it. That is a different point. But if that is his view, the correct course of action is to pay out (as he is obliged to by European law) and then sue the government to reimburse him.