Probably because no preventative is perfect. News reports indicate that this was an intelligence blunder first and foremost. I would look at intel services before I complained about laws which seem to mostly work.
Also, read over at TTA that France is #11 per capita in gun ownership, so I'm not so sure your adjective "strict" applies anyway.
Also, read over at TTA that France is #11 per capita in gun ownership, so I'm not so sure your adjective "strict" applies anyway.