Why is there a fee for the auction house?

If someone could please give me one good reason as to why 2k needs to take away from us earning a make-believe currency?

This game screws you over in every other way imaginable from unreasonable pack odds to making sure natural rewards and progression are so slow that you feel the need to buy VC, and yet every time I read that message when I go into the auction house it just feels like an extra spit in the face.