Second Life as always advertised itself as a "real" economy, and people make real money in it; Linden Dollars, the virtual "money" used in Second Life, is freely exchangeable for real US dollars.
Therefore, Second Life is a legitimate revenue source in real money for people who sell goods and services over the system, and taxing it is just as appropriate as taxing the sale of any other goods or services.
World of Warcraft money can not be exchanged for real money (people buy in-game virtual money from cheaters, but there's no way to turn the virtual money back into real money, and Blizzard shuts down the accounts of anyone caught cheating by buying in-game money), so taxing WoW money makes less sense.