It's for rental scooters, not all scooters. My guess is two reasons:

1. They're left everywhere and can hog probably already crowded Paris sidewalks and bike racks.
2. They're mostly used by inexperienced people who don't know what they're doing and will ride on sidewalks with little regard for pedestrians, especially in Paris with the large amount of tourists, who the parisians already hate.

I'm making an assumption based on my experience in a smaller American city but they are a nuisance and left anywhere and everywhere. People like to throw them in bodies of water too.