Pretty sure that they are scared that GW2 is going to make them bleed more subscriptions.
While it obviously won't kill them... the temptation of a FRESH and GOOD "Pay for box only" / F2P business-modeled game, will probably suck quite a few players from WoW... and they are already bleeding subs even without GW2 as competition. (People are finally getting sick of WoW)
So, this is obviously to prevent some of the bleeding, IMO.
The really funny thing is... Arenanet was created by a faction of ex-blizzard devs.