Because Saint-Louis and the rest of the Azur Lane Prototype ships are part of an ongoing WoWs×AL collab.
The tag was only added to a handful of AL images though, and I don't really know if it should be added or not.
Unless there's anything in the image that is specific to World of Warships then there's no reason to tag the post with it. Azur Lane characters are Azur Lane copyright. A collab with WoWs doesn't make them WoWs copyright.
Unless there's anything in the image that is specific to World of Warships then there's no reason to tag the post with it. Azur Lane characters are Azur Lane copyright. A collab with WoWs doesn't make them WoWs copyright.
Rather than debate it with you here, you should bring it up in the forums. I don't believe it was ever officially decided if they should or shouldn't be tagged WoWs. I have my opinions, but I'd rather have additional input.