Since WoW isn't freemium (yet? or never) it's probably a pserver looking to make some cash.
Freemium WoW is not something Blizzard is 'spending a lot of time thinking about' | Joystiq
WoW Team Doesn't 'Have Any Opposition' to Freemium Concept - IndustryGamers
Both articles imply it's a whiles down the road for WoW - probably at least 6 months - 1 year since they just introduced the annual pass, so IF it's in the picture soon, it'd either come part way through and the pass would go towards the freemium perks or the freemium model would be introduced after the annual pass buyers' time expires.