I don't know what he was thinking. If they warned him in advance, and he still went to North Korea, then he's a prize idiot.
I am not aware of a rule that says you cannot give lecture about a technology which is publicly available and if US is having economic sanctions towards a country you cannot have any business with them is strange, i am not aware of any wrong doing by Virgil Griffith other than he went for a blockchain conference, now it is evident that his plan to go to North Korea was not a wise decision as he will be spending a long time fighting to clear his name.
This kind of behaviour brings cryptocurrency into disrepute.
These kind of behavior will bring shame on the nation for arresting someone without much proof and how the government wants to control every individual.