This. Games that employ systems that allow you to buy monthly subscriptions with in-game currency or trade cash shop items for currency have little to deal with in the way of RMT because there's literally no market for them. Why risk your account when you can do it legitimately with no risk?
It does carry with it some pluses and minuses, of course. Personally I don't care either way, but the option of being able to earn your monthly subscription through in-game means would certainly be an attractive prospect to many people, and the added benefit of being a means to get rid of RMTers once and for all would be nice.