So lets say we find Cutesquirrels in Port Sandy with 200mil to it's name. Suddenly the character logs on, sends a tell to Xcloudsephirothyxxx about the trade being ready and to meet them in said zone. The characters meet up and Cutesquirrels gives him 10mil through trade, immediately also logging off afterwards. Imagine if SE adopted a zero-tolerance policy that was shaped around the events of an RMT-transaction? Oh man. The legit players' line of defense would be something like "if I were muling, I'd have to log off my main character first." Also, I'm not sure how many people would have gil-only mules. =\
Problem with that is just as you pointed out, there are enough *legit* players who do this that you can't peg it automatically as RMT behavior.
I have 100+m on a mule, so do several people I know.
(Helps keep those incredibly expensive impulse buys to a minimum).