This is a tactic Ryanair have been using for at least two years. French ATC strikes are a godsend to them, as it allows them to cancel almost any flight that goes anywhere near France, and they have an 'external influence' to avoid compensation claims, whilst saving pilot hours to crew future flights.
What they don't tell you is that they don't train all crews to fly Oceanic routes, so whilst every other airline can fly around French airspace on the Tango routes, they can't. Why would they, it's more cost effective for them to cancel the flight than increase fuel burn and pilot time during times of strike.
All just my opinion, of course.