I haven't used the neutral GTM yet but, doesn't it cost more to use it?
If so that would be a "tax" on using it, if you were allowed to just send credits you wouldn't be charged the "tax". Although they could put something in to charge that "tax".
The we are at war thing also applies. During war time a person sending money to the other side would be seen as supporting that side. Since you can't change sides in this game the only way to restrict supporting the other side is to limit what interaction you have with them.