Flight Global reports that the trade body IATA is "furious" at the European response which, it is claimed, was based on incomplete or unreliable information.
The regulation of civil aviation is a complex matter. There is the International Civil Aviation Authority (ICAO) which sets basic international standards and practises. The European Union has a Transport Commissioner. Numerous other agencies also play a role (e.g. The European Aviation Safety Agency (EASA); Eurocontrol) as well as national governments (Dept. of Transport), national regulatory bodies (e.g. Civil Aviation Authority) and providers of services such as National Air Traffic Services. It is unlikely that a decision to close down national airspace could be lawfully taken at a lower level than national government.
The pity is that the "blame culture" is now swinging into action. Decisions, based on safety, have been taken in good faith and the volcanic activity still continues and, for all we know, may worsen. The important thing for now is that those responsible make safety assessments on whatever evidence is available and not be influenced by financial and political considerations. There will no doubt be a lot of "lessons to be learned" but they will need to await another and calmer day.
See also London Volcanic Ash Advisory Centre. For those interested in the technology have a good look around this website. Interestingly, Switzerland has been using a laser-based technology to monitor for volcanic ash. The Airbus website carries very useful information for aircrew. There are over 1500 "active" volcanoes on earth.
Meteorological Office response to Civil Aviation decision to change the engine tolerance levels for the safe levels of ash ingestion into aircraft engines - Met. Office 21st April.
The Times 21st April - Lord Adonis states that the airlines did not force the government's arm.
Addendum 22nd April: It appears that the "budget airline" Ryanair is refusing to pay the costs of passengers who were stranded - see The Guardian 22nd April. However, later in the day, there was something of a reversal of attitude - see The Guardian 22nd April.