What does this even mean? Illegal just means the other team says they can't do it. I can't believe that you're very surprised that our team told their team no. NK is almost universally regarded as a bad actor.
Ideally the law provides equal protections and rights to everyone. International law is much further from this ideal than US National law, and many people are surprised to learn details about international law.
If it's equal, why can we do it and they cannot?
That's kind of the point. International law isn't equal, there is often an 'us vs them'. The two are very different realms.
All law is written by crooks for their own benefit. National law has 'us vs them' baked into it just as much as international law.