I understand that the fines, as high as 10,000€ per apartment rental, have been levied against the owners, not necessarily those who simply list apartments on their websites. However, as Airbnb is involved in accepting rental payments, they are responsible for forwarding, to the city, the taxe de séjour. As Airbnb has not collected this tax, Airbnb may be required to come out of pocket with a sizable chunk of change to cover their tax liability.

This is currently being discussed and exactly what Airbnb ultimately pays is a big question.