I don't think they use them as mules to send the gold, but rather as mules to send mats to and sell on the AH.
I just took an account back from a Chinese company the other day after renting it to them. Had a level 1 toon on a server that didn't have any other toons on it. Literally had netherweave bags FULL of JC stuff (tons of blue gems etc) and a shit ton of Titanium Bars as well as 2k gold.
The reason they do this has to be b/c of the restrictions placed on trial accounts. I'm surprised trial accounts are even allowed to level nowadays w/ all the restrictions Blizz has![]()