That's a silly way of looking at it. By leaving it a while for them to accumulate and sell the gil, they make enough money to continue buying new accounts. If they were very fast in banning the RMT accounts, the accounts would end up losing the RMT money and they would either change their tactics or go broke (hopefully the latter).

Put it another way, RMT is watched for 1 month, over that month they make 2m a day so say 60m gil. They sell half of it at... /checks chat log.... $5 per million, that is $150. Account gets shut down and they buy another game for $50 and still have $100 profit.

Ban them before they can sell the gil, they still have to buy the game but they haven't sold any gil to fund that purchase (eventually they will ban all the accounts the RMT steal....).