it's just a problem that no server and their staff want to deal with. People can lose their entire account or all their items on it. It's easy to just ban people who try to CST in order to prevent potential problems with it. Trading would probably be illegal if it was even plausible to crack down on it and handle incidents when people had already been scammed. It's way too messy and impossible to deal with, CST on the other hand is easy just to ban people who try it.