There were three choices:
1. Blockade only. Millions starve. Russia enters the war and takes a chunk of China, all of Korea and half of Japan. Not a good outcome.
2. Invade. Millions die on both sides. America goes bankrupt. Russia enters and the result is worse than #1.
3. Nuke those cities. End the war. Lose North Korea. Millions live. America doesnt go bankrupt.
And people getting their panties in a bunch about nukes need to read about the Tokyo firebombing (and all of the other fire bombing) and some of the night bombing of Germany.
War sucks. Total War is a bad thing. Sitting around 80 years later and passing judgement is pretty easy.
Or as Archie Bunker would have put it, "Would you feel better if they was pushed out of windows?"