If SE wanted to, they could just run a report on the database to find if the service account of a friend matches the friends list owner. Then bulk ban and increment the RMT task force "dealt with" numbers. SE has confiscated all gil from innocent accounts and let the gil buyers keep the bought items and keep playing, so I won't try to predict whether they would actually ban anyone for this.

I don't know why SE doesn't allow it, but I also don't understand why alts would be preferable to more storage or extra retainers, especially since bound or untradable items couldn't be mailed anyway.