Why are people celebrating this?
1) Because they're French. They killed millions of Algerians during their 100 years of colonization in Africa. Their First Indochina War, which lead to several hundred thousand more deaths, lead directly to US involvement in Vietnam once they retreated (typical).
French colonial empire - Wikipedia
2) Because they're Christian, even worse - they're Catholics. Medieval Catholics. One of the worst time periods of one of the worst religions. The Inquisition started in Medieval France, and was basically the precursor to the Holocaust.
You can read what Malcolm X had to say about the detrimental effects that Christianity has on pacifying the masses in his Autobiography, or his interviews.
3) Because this wasn't even an arson, so I don't think anyone got hurt. It happened on its own. Some could even say it was an.... act of God.