Questions regarding meso buyers

So I understand why GMs ban botters and hackers in most games (exploitation of game code that could be taken further, unfairness for legit players) but what are the reasons behind banning meso buyers/sellers? I mean, assuming that those websites don't give you keyloggers. Serious question. At first I thought maybe it was because of legal issues (i.e. technically the mesos/gold don't belong to the person, so they can't sell it) but I wasn't sure, or if there were any other reasons. So exactly what is it about real world money and ingame currency transactions that's so damaging to game worlds?

I hope this fits this section... I mean, it involves meso sellers and buyers and all.