They registered warlordsofdraenor.com
They never registered BurningCrusade.com
They never registered Cataclysm.com
They never registerted MistsofPandaria.com
In fact they only registered one domain out of all the 4 expansions and that was Wrath of the Lich King.
In fact MistsofPandaria.com was registered as late as 1 month ago by a person in Korea.
So why would Blizzard suddenly care to register expansion name for World of Warcraft, I don't see it.
** Also you have tablet PC's and such small devices that World of Warcraft expansion can never exist on, under the trademark.
** While The Dark Below has not, while none of the previous expansions has that under their trademark either.