Because this isn't a service from one player (or generally group in this case) to another player. The person paying is contributing just as much as the person being paid. Rather than being like selling an item, a more accurate analogy would be trading an item for another item of equal value, and then on top of that charging the player you traded with for having facilitated the trade to take place. If it's an even trade, then you shouldn't have to charge on top of that.
