Except that now, in South Korea, it is. A ruling by the nation's supreme court has stated that virtual currency is the equivalent of real-world money. Even more sweeping are the details of the case that led to the ruling, in which two men were on trial for exchanging a large amount of Lineage II Adeena for cold, hard currency.
For those of you who might not be drawing the link, the core there is that selling in-game currency for real money is essentially just an exchange of currency and perfectly legal in South Korea. This could have sweeping implications for RMT operations the world over, not to mention free-to-play games and... well, online games in general. The official story is available online from both the Korea Times and JoongAng Daily.