So what happens if I received my Dan grade from Japan or some other country where the state is not involved and move to France.
In Germany it is illegal for a scientist to call himself Dr. unless that Ph.D. was from a German university or an exception applied for and granted. I just can't fathom the thinking behind this - who comes up with these laws and why.
I am not sure if this is true for France, but I have always had a suspicion that it is:
One reason I could conceive of for a country to decide to sanction and control the rankings for practitioners of foreign martial arts would be if public facilities and perhaps funding is made available to people for practicing martial arts. You might not want someone with a black belt from a belt mill to come in and say "okay, I am going to start using the gymnasium over there with the sprung floor to teach my students, I will need a couple of two-hour blocks a week." And whatever else they might demand, I am an American so my imagination does not serve me very well here.
But the point is, IF (and not sure if this is the picture in France, but I think it is something like it) the people decided to set aside a bunch of public funds for facilities for martial arts instruction, there would have to be some kind of official vetting process for people who were going to teach there. And maybe you'd want to do that INSIDE the martial arts system you were trying to support as opposed to externally.