Bunged in the slammer is the solution to all things paper work related in France if they find an issue. They know fine you will be off sharpish and won't return. So they hold you until its sorted.
Then they stick you in the local sheriffs court PDQ. And then fine is delivered. You can bitch and complain as much as you like but you won't get the aircraft back until you pay it or complete the legal appeals process which takes years. Once you pay it to get the aircraft back you can kiss good bye to the money. I think there is an option to take it to a higher court but the aircraft stays where it is and you are open to a larger fine so take your choice.
It doesn't really matter what nationality you are apart from French.