Precisely why it will likely incur bans still, as there is a risk of a player being defrauded.
If they did want to allow it and intended it to be, you would be able to open a trade window between the to two games, so the trade can be undertaken simultaneously without the risk.
Also look at the statement they made about it people - one of the primary problems they mentioned is the ability of gold trading to massively disrupt the game economy.