Helicopters are blind ....
Yes the helicopter probably doesn't know whether it is being flown VFR or IFR ...
.... I am sure the ash would have the same effect on the engines whether the pilot is flying VFR or IFR.....
The restriction is only for IFR though....
Presumably because if you are looking out of the widow and free to adapt your path there is no need to fly through the dust particles ....?
(It seems rarer for twins to have particle seperators though ... is that true?)