Why NK can act only rationally, while US could choose to act irrationally? Both sides can act irrationally, as history shows.
This is, after all, what American presidents have been doing for the past few decades.
1. NK is not expansionary.
2. NK has nukes. US does not want anyone to use nukes, even a little one.
3. Any attack on NK means many deaths in SK.
4. Bad press.