Mechanical Turk Masters (mechanicalturk.typepad.com)

Mechanical Turk is now trying to charge requesters extra for the ability to send HITs to a select group of Turks they call masters. Amazon usually focuses on the platform, and no so much the middle-ware; so why are they different in this case? I think the companies already providing layers on top of Mechanical Turk do a pretty good job managing the quality of their workers.