Did they change Darkmoon Gift Boxes from BoP to BoA recently? If not, so the boxes are BoA since years why do you think this is not intended?
Edit: Actually I found many posts from Mist of pandaria on wowheadf stating it's not soulbound. Like this:
http://www.wowhead.com/item=93724/da...nts:id=1821758
The box you get from dailies has a name of "Darkmoon Game Prize" instead of "Darkmoon Gift Box" you mentioned. They aren't BoA either.:
http://www.wowhead.com/item=93724/da...prize#comments
So you can trade, sell and mail them since... were they ever soulbound?
I still don't see any exploit here. It's pretty much intended since years.