One of the main reasons they are not allowing the items to be traded after being purchased is the after market effect it would have. Knowing there have been quite a few dupe methods since WOD has launched gold has plummeted in value and websites/dupers/currency flippers would just buy them and re-sell them for less than blizzard is asking for their normal gametime vs real world money.

Its not really anything to do with the economic impact it simply is a measure taken to prevent this third party market. They wouldn't really care because people are buying the tokens at their price to begin with but it subsequently would possibly have very harsh repercussions on the sale of game time to those who actually pay for the game themselves. Why buy from blizzard for $15 when you can go buy a token from XYZ site for $8? If one doesn't want to spend their gold and would rather use $.