Well, you can also say that the highest level of safety is not being maintained as long as there are planes in the air.
But there's a difference here: the general risks of aviation are well understood and quantified, whereas the risks of volcanic ash are not. It's clear that volcanic ash is a bad thing, but the maximum allowable ash concentration for a given level of safety is unknown. In aviation, if you don't know, you don't go, which is what has made air travel so safe. In this case, that general guideline is being ignored, for the sake of money.
If flying through ash means bringing pax to their destinations 3 days earlier, it pays off even if one plane in 10000 crashes because of a higher danger level.
You realize that one in ten thousand means a dozen or so crashes per week, with more than a thousand people dead, right?