The UN is an American creation. It's headquartered in New York City, so that the third world has a voice in modern western culture. If it wasn't for the UN the Cold War would have blown up into WW 3. Gun laws and the U.S. Constitution have a jurisdiction above and beyond the UN Charter.
Personally, this shouldn't even be an issue. It doesn't mean that one person decided to shoot some place up doesn't mean the others 3 million of us are going to fallow in his foot steps. Men fought and died for these rights. When they gave us the right to bare arms, it wasn't for hunting or fishing. They gave us the right to bare arms so that if our government got out of control, we will have a way to stop it!
Because they are a bunch of third world countries that are jealous of the US lifestyle. Like most losers they want to bring others down to their level instead or rising up.