I know sometimes cartographers leave nonexistent landmarks on their maps to catch plagiarists blindly copying them. Not sure if that's something similar here.
Could also be a method of identifying information leaks. Leave unique imaginary landmarks on different prints associated with certain people or groups of people that received that print.
You're right that this happens. However, if I had to guess I would say that it's due to bad information / intel.
North Korea is a closed country with limited access to information (think before the internet). On the flip side, I don't think catching copyright violators would be high on the list of concerns of the people making these maps.
Could also be a method of identifying information leaks. Leave unique imaginary landmarks on different prints associated with certain people or groups of people that received that print.
Or just wrong info. Who knows.