As others have pointed out, there was obviously a problem with the rules. There is never zero volcanic ash in the atmosphere. We have always coped around the world until an invisible plume hit the UK. Thank heavens we have leaders like Captain Willie Walsh to sort things out.
Don't forget that WW is effectively responsible for the zero figure due to the airlines long standing refusal to provide provide an alternative figure.
He is also partly responsible for the chaos as he must have been fully aware of the risk of an Icelandic eruption and the potential effect it would have had on the industry - but did nothing to mitigate it until after the issue became a real problem. That indicates piss poor planning.