Maybe we should wait to find out, there may be fireman inside making sure there are no people , evaluating the situation, I assume you need a strategy and not just spray water where you see flames.

They might be more focused in evacuating the place and making sure none of the surrounding building catch fire. Paris is a huge firetrap, you really don't want it to jump. At least it is quite isolated from the rest of the city.

Update: In the news they said that firefighters are entering the building to save as much art as possible before spraying water.