http://i.imgur.com/qaS4BOz.jpg
Oh god, leave it to some government drone to put a 'NOFORN' classification on a Top Secret document, because just in case one wasn't clear, Top Secret documents shouldn't be shown to Foreign Nationals. NOFORN pretty much only exists on it's own in training military/fresh government drones of how to handle actual classified materials. Labeling something as 'NOFORN' is like putting a 'caution: hot' warning sticker on a flamethrower, or a no smoking sign inside an oil tank.
That said, I believe there was real concern that even with the 'tools' the NSA has, games like SL use encryption for chat that is in part based off the software of the game itself. There's no website you can hack into to get WoW chatlogs, you have to be on WoW. It would make sense for people plotting things to communicate with such a game, but I'd assume SL and WoW would just be too American in that regard. Maybe some game more Russian or Asian would be the means.
So they have to be in SL, and they might as well blow taxdollars doing pointless research while they're there. Also, from the outside, SL might look like an easy way to launder money, with it's own currency, but I believe the interaction is one way mostly. Real money => Lindens, not vice versa. I could be wrong, but SL is run very much like a ponzi scheme in some ways.