I support conservatism in these matters. I am sure the risk in this case is small but not zero.
Qantas does not need another reason to give it another PR
nightmare.Neither do Tiger.
If you dont fly into ash clouds you dont get the damage to hardware, bottom line or bad press. Just imagine an engine going tits up (no matter the reason) on the way to Melbourne or Adelaide. Damned if you do or dont. At least by not flying risk is diminished.
Airlines make their own calls. It is their business.