We've received numerous tips about this, and I'm sure the folks over there would be touched to know that you all care so much. It's likely that the ad came down through their parent company zam.com – so don't read too much into it.
But it is rather funny seeing a large Warhammer ad on a WoW site. These ads have also been known to exist on WoWWiki. They too likely have their ads chosen by their parent company Wikia, just like AOL does for WoW Insider.
So enjoy the Warhammer ads on Wowhead for the time being. You can learn more about Warhammer over at our sister site Massively.