ETQW was a bit different than other id titles in terms of technicalities and legalities.
As far as I know, it uses a proprietary authentication system which is still based on a service provided by Demonware, a subsidiary of Activision.
The game was licensed to Korean publisher Dragonfly by Activision to be released as slightly different exclusive for Korean and Japanese market under the brand of Quake Wars Online.
As you see, instead of clearing the legal obstacles, ZeniMax/Bethesda simply unlisted the game on Steam. Thankfully, it is still functional online. Besides, retail copies are quite easy to get.