As I wrote once before, it’s sometimes hard to know which public lawns one can walk/stand/sit on in France, and which not. There doesn’t always seem to be a logical reason (that we can see anyway).
Here’s the original article,
So, we just follow what others are doing, and enjoy the grass when we can.
Here are a couple more of the warning signs—there seem to be endless variations.
Enjoy, and let me know if you’ve seen others.