Laws without enforcement are ways to placate the citizens. If he intends to enforce this law, he needs the cops to do it. And then if he doesn't bow to the pressure, it's not inconceivable that a French leader decides in the best case scenario to enforce the same laws on other religions for fairness.
So it's the spiral Daniel Greenfield layed out; you can have muslim immigration but then you need a police state. Then eventually Islam looks less horrible in comparison and you get more sympathy for them and then you move out of stage 2 jihad into the annihilation stage.
Kay you muslims, bad. You best sign that you don't want to take over France like you openly say and all the money you're getting from terrorist groups, well you sign you don't get that too. And we're going to have a council that we'll pretend has the authority to decide who is an Imam because we are reformers totallyy. Your ideology says to establish a super state, well you just sign now that you're just about being French and not that.
So you bring a snake into your house, the snake bites, and therefore the Elite-Smart psycho solution is to make the snake signs that it wants to live in peace with the rest of the household. And you will establish a council of snakes to declare biting snakes as not officially sanctioned snakes. I'm sure the muslim world is shaking in their sandals.