Hecklers veto. If police are concerned about an attack on the protests, they should beef up security around the protests, not block them altogether.
In reality, Paris just wants to suppress dissent. So do you.
And that beefed up police presence worked well for the two nuts who did the boston bombing right? Remember that unlike those of us who live in Canada or the US the laws on the books are fundamentally different in many parts of Europe.
In reality, I'm playing the devils advocate. You're just making an ass out of yourself.