The entire country is under arrest.
They are all brainwashed from birth to believe that members of the Kim family are literally deities. Citizens of the DPRK can be sent to prison for not keeping the mandatory portraits of Kim Jong Un's father and grandfather (that hang in every home and public place) sufficiently tidy, or for blocking the audio that plays from the government radio that is in every home.
The DPRK spends a fantastic part of its GDP maintaining the Kim family cult of personality. It's amazing that even with all that, Kim Jong Un is afraid he'll lose control if he leaves for a day or two.
Didn’t he leave a little while back on a train trip to Beijing?