Heck, that's not even a crime here... We let newspapers publish national secrets with impunity. We let anybody revise and rewrite history any way they want to, and publish it if they want to and can find someone to publish it for them. We have children's books pretending to be non-fiction that paint Cuba as some kind of cross between paradise and the Emerald City, sans midgets.
I'm all for ridiculing anybody who tries to deny Nazi brutality, but charging them with crimes in a court? That's one way of legitimizing and promoting those ideas.
It's also one way to have the entire sorry episode of french collaborationism and the Vichy sell-out dragged out into the open again. This is exactly what anti-Semitic france doesn't want the world to be reminded of -- the fact that they helped the nazis exterminate the Jews. Many of those collaborators are still alive and Le Pen will be certain to introduce this in a trial.