The Trump administration is banking on the plenary power doctrine to have its travel ban approved by the Supreme Court.  However, that doctrine doesn't grant carte blanche authority.  Furthermore, the plenary power doctrine has its roots not in the constitution, but in racist immigration laws of the 1800's.  That's why courts have limited the effect of the doctrine.  
Instead, the travel should be evaluated based on its ability to reduce terrorism in the U.S., as Trump says it will.  However, the addition of North Korea to the travel ban shows that the purpose of the travel ban is not to reduce terrorism.  North Korea is a closed country, and almost no North Koreans visit the U.S.  Those few are largely diplomats going to the U.N., and they are allowed under the travel ban.  There have been no terrorist incidents in the U.S. caused by North Korean immigrants.  
Apparently, Trump added North Korea to the travel ban to make it appear that it wasn't a Muslim ban.  It's still racist.