The sanctions imposed by the US government describe explicitly what is and isn’t allowed, and there is a very specific law mentioned in the article.

  Griffith was arrested and charged with conspiracy to violate the International Emergency Economic Powers Act (IEEPA) in November 2019 after he travelled to North Korea to give a talk on blockchain and crypto.

None of this is post facto - he was warned what he was doing violated a law, did it anyway, and is now facing charges for violating that law.