That's amazingly naive, Jacques. The same argument could be used for embassies worldwide: they are considered part of another country and so on. Yet embassies are routinely tapped by everyone. Been that way for decades.
The UN is a treaty organization, not a world government. It has as much sanctity as any other treaty organization -- not much. If folks want to construe this as being something to do with them being hosted in the US, then I'm at a loss as to how to continue the conversation. The Chinese would be okay tapping the communications but the U.S. not? Doesn't make any sense.
A little bit of context here is all I ask. Intelligence agencies are supposed to find out things about diplomatic, political, and military matters. They break and bend all kinds of rules doing so. This will continue whether or not you or I agree to it, whether it's the US or the Russians, and whether the UN is in New York or Botswana. I'm fine with that, mainly because the more each country knows about the inner workings of the others, the better they can predict reactions to political moves. That makes the world safer.
I think we've just reached the point where somebody says "NSA" and the mob all gets out the pitchforks and torches. That's a shame, because there is a very serious problem here, and it has nothing to do with the U.S. or the NSA. It has to do with technology making the entire life of the civilian population available to anybody with the appropriate political power. Not limited to NSA. Not limited to the U.S.. It's a worldwide problem based on the tech we create everyday.
We should really get off the NSA jag and on to actually solving problems instead of this emotional overreaction to any story with NSA in the title business the MSM is pushing us into.