The reason was that he was acting like a suspicious cracker from a country that sends almost nothing but suspicious crackers there. 95% of Americans who go to the DPRK are either feds or fed associated journalists who want to smear the place. So this suspicious cracker breaks into a military section of a hotel, is told to go back to his room, does so but then sneaks back to the military wing to do something fucky with a poster.

They were really harsh on him, but acting like a spy while being the exact type of person who would be sent to the DPRK as a spy isn't nothing. The guy was an idiot.