This isn't vague or arbitrary. The sanctions against NK are very specific, and lengthy.
No we are not. The constitution specifically says that only congress can declare war, and congress has made no such declaration.
Edit: I feel bad about being facetious, but I feel like the point is relevant. The US, in a practical sense, does not need congress to ok war. Pointing that out as a reason to doubt the basis for strict treatment of North Korea is like pointing out the speed-limit is 55 when someone travelling 56 gets T-boned.
Do note that the US never declared war on Iraq, and that did not stopped it from fighting two wars that were very real, both of which with the support of an international coalition.
However, the United Nations did declare war on North Korea, with US support, and the US is the major participant in that war; and no peace treaty was ever signed, merely a cease fire.
The truth of the matter is that declarations of war are patently irrelevant regarding whether a nation is deemed hostile or not.
You write as if Congress has not weighed in on this matter, simply because it has not declared war on North Korea. In fact, current North Korean sanctions were codified and authorized by Congress in the North Korea Sanctions and Policy Enhancement Act of 2016, which expanded upon the sanctions regime that had already been in place. Virgil Griffith’s alleged acts are not illegal because of a vague characterization of North Korea as being “hostile”, but because Congress explicitly made it illegal to travel to North Korea to give technical advice.
There is no ambiguity in the classification of North Korea as a hostile regime. Not only there is the fact that the Korean war never ended with a peace treaty an only had an armistice, there is also the hostile and recently degrading relationship that in 2020 escalated to the total elimination of official communication lines between north and south Korea.
I should point out that there is nothing vague about sactions.
https://en.wikipedia.org/wiki/Sanctions_against_North_Korea
If a country like the US imposes sanctions on a country which includes, quite prominently, restrictions on investment and financial activities, and afterwards you go out of your way to provide assistance on how to circumvent those restrictions on investment and financial activities... Where's the surprise that your assistance gets you in jail?