There isn't a reason to omit it, but I could personally care less about its absence. I'm going to enjoy the game no less, regardless of what flag is and isn't on the cover. If you really are going to make an enormous deal about it, write to WotC, see if that'll help.
I seriously doubt they'll change their designs because I complain.
And I'm not making an "enormous" deal about it. This is a thread about the box art for the game, so posting a couple of opinion points here is hardly making an enormous deal, is it?
Doesn't mean all Frenchman around the world just stopped fighting.
Well, most Frenchmen, and Poles, and members of other occupied countries who got the chance to fight on conventionally did so because another country continued fighting without being occupied. Who was that country? It's on the tip of my tongue... ah... nope, it's gone. Airbrushed away.