Because Empire is a very terrible thing to Communists - the most evil kind of country on earth. That's what they want their starving population to believe so they won't question why their lives are so miserable.
The North Korean government has it's clueless citizens convinced that Americans stay awake each night thinking of ways to destroy their great country - even though it's of little consequence to America with the exception of being a regular nuisance. The people simply have no way of knowing otherwise. Sad.