It's because "Mairie" means "townhall", wich is describe by the tag amenity=townhall, so these names are useless. 
I deleted them after a discussion on the French OSM-Forum (forum.openstreetmap.fr) 

But, I did some mistakes in this changeset : I did it too fast, too big, and I take accidentally some objects who are not townhall but have the name "mairie", like bus-stop. This mistakes was reported by the contributors of this forum, and they reverted this changeset (https://www.openstreetmap.org/changeset/139605572).

If you want more details about this discussion, you can go on this page :  https://forum.openstreetmap.fr/t/utilite-des-noms-descriptifs/16715

Have a god day,
La Voie