French people are famous for over-legislation. The lighting on the monument is considered a work of art which is subject to copyrights. Taking a picture of the Eiffel Tower at night is not illegal, but distributing these pictures for professional purposes requires permission from the Societe d'Exploitation de la Tour Eiffel (SETE).
Without SETE's consent (which likely involves monetary compensation), distributing the image would be a copyright violation.
Pictures of the Eiffel Tower in the daytime are not subject to copyrights because the author of the work (Gustave Eiffel) died in 1923. Copyrights extend 70 years after the death of a work's author. Therefore, copyrights on photos of the Eiffel Tower ended in 1993. The lighting however was set up in the 80's so the copyright period is still in effect. Also, it looks like SETE claims new copyrights for each major change in the illumination of the tower.
Here's the official statement in reply to the post of Wikimedia Commons featured photo.
https://commons.wikimedia.org/wiki/Commons:Featuredpicturecandidates/File:EiffelToweratNight.jpg
