In MMO game loots are distribute randomly to player and market use to satisfy these distribution.
As for now AH price and private trade limitation cause market to be inefficient. Pricing limit causes some item to be trade irrational range which discourage player to trade via AH. Somehow the need for trade did not satisfy and need other option to trade, RMT.
They could be legit player who cannot utilize trade provide by system and decide to jump off and use illegal mean to satisfy their trade if they have enough fortune. It could be 5% 10% or even 20% off user base that are doing these activities.
We are hope that IMC would tackle issue not only kill those illegal trade but also provide more efficient, more flexibility trade system.