bin Laden sure didn't sound aware of that when he criticized the UN.
Oh please. Where else would he ever find one place where the U.S. can be criticized with impunity 24/7? And it's not even our property, really.
Besides, as a target, the U.N. is relatively teensy. It's not tall and it's not big. Flying low enough, across Queens and the East River, to crash into a short little building like the U.N. would be problematic. You'd be just as likely to hit the FDR Drive running underneath it, or overshoot and hit the taller buildings behind it instead.
Actually, you could probably lob grenades by hand off the roof of the U.N. hotel and have an equally good chance of success. Plane not necessary!