To me closing the entire airspace is a CYA move. Best way to prevent accidents is to not let anybody fly..
However, like not all humidity and low temperatures are "icing conditions",there must be something about the concentration of ash particles that determines whether or not it becomes a safety hazard.
Flying VMC/VFR means you should be able to "see and avoid". To prevent the general public from flying VFR is something I can understand, but to totally ban SAR/HEMS on the basis of perceived safety/unsafety seems excessive.
Especially when helicopters are suited to land at the merest hint of (engine) trouble, with no doubt a highly alert crew under the circumstances.
But hey, I am just a pilot..apparently I cannot be trusted to make sensible decisions in todays PC world.