For starters, it is legal. Deciding to disrupt French economy by other means (like sending hundreds of fake anthrax letters containing small amounts of baking powder and some threat in broken english) would probably do more damage to their economy, but it wouldnt be legal.
Then you'll have to take into consideration that the public support for Chiracs policy is rock solid. I think something like 80% of the French public supports his line of action in the Iraq question.