Is there any other sub-game that has this restriction? Or such a hugely restricted mail-system in general?
Complaints about the level of RMT pop up with depressing regularity here, so I can't help wondering if this is hindering it in any significant way. Even if it is helping to prevent RMT, inconveniencing a lot of customers who abide by the rules is hardly an ideal solution.

As others have said above, it would be really nice to see an official response on this.