From the article:
It is not professional and they see it as an insult to their dignity
It is very much professional to observe the cultural laws of a country you are visiting. Frankly, deal with it.
As the aircraft is considered "French" soil, I too would have a problem being told to wear it onboard the aircraft. However, once you step onto another someone else's soil, you are required to abide by their laws - if this is one of them, so be it. It's not for you to determine if their law is unjust or not.
I would suggest that if they feel so strongly, they don't bid for the run or call in sick the day the schedule shows them going. But don't hold the airline to ransom when all they're trying to do is protect their staff in a country that is vastly different to some of the ones they routinely go to.