Joe in time of war to protect your own civilians (In the case of the Korean War the citizens of South Korea) the UN were perfectly justified in December 1951 shifting the focus of their strategic bombing policy to reflect hitting targets inside North Korea that would reduce the ability of the North to move hundreds of thousands of Chinese troops through their country and down into South Korea. That meant hitting airfields, transport hubs, storage facilities, etc, all of which were to be found in Pyongyang. In fact Joe have a good look at a map of North Korea and see what a strategically important hub it is, then you might see why the UN decided to attack it.