Engine manufacturers have (finally) decided on a 'safe' level of ash ingestion and governments are applying regulations based on the 'unsafe' ash forecast areas.
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Now a 'safe' ash tolerance level has been arrived at with the engine manufacturers and airspace closures made in accordance with it.
Except that, in the mentioned case, France has not acted in accordance with the 'unsafe' ash forecast areas, while others have. Some operators have cancelled flights into/out of the region, others haven't.
I imagine a lot of pressure on the shoulders of a pilot who informs his operator that he doesn't wish to fly into an area of forecast 'unsafe' ash if the operator has decided to fly there as long as the airspace remains open.