There was some international law. If we were to just keep them, we'd be pirates. Yes, Yemen "promised" not to buy them, but they did. Unfortunatly, they didn't break any written agreement.
When it comes down to it, there isno international law. The world is an anarchy by definition. Every country gets to do whatever it wants.
Of course there are consequences, but the name of the game is the balance of power. It is a zero sum gain. In order for one nation to gain power, another must lose some. Creating alliances with other powerful nations is the only way to multiply one's power. And that is exactly what NKoreaChina is doing.
It is interesting that the US recently made the official statement that we would defend Taiwan if she is attacked. That commitment now ensures that we can be weakened. If we are engaging Iraq, all China has to do is lob a few across the Straits, and we will have to respond.
This is getting goddamn scary. There is no relief in sight.