You can easily debate whether France should be taking military action against ISIS without claiming that ISIS is right.
This is like saying that because France has laws against gender discrimination, so you could not advocate oppressing women as a show of support for Saudi Arabia, it is now impossible to argue against going to war with the Saudis.
What the law does do is make it harder to instigate more terror attacks within France.
You can easily debate whether France should be taking military action against ISIS without claiming that ISIS is right.
This is like saying that because France has laws against gender discrimination, so you could not advocate oppressing women as a show of support for Saudi Arabia, it is now impossible to argue against going to war with the Saudis.
What the law does do is make it harder to instigate more terror attacks within France.