Right. It makes the chain of command....
In military terms, if you're going to prosecute a mission, you like to have the same chain of command responsible for all aspects of the mission. When you talk about a single vehicle or a single ballistic missile coming in, NORAD would have a portion of that mission—detection and attack characterization. USNORTHCOM would have the defence part. Then, if anything was to land in North America, NORAD would pick up the assessment and the detection of a nuclear detonation if there was anything happening.
That describes the complexity when you go from one command to the other, and go back to a command, in order to prosecute the same vehicle that is now flying through space and approaching North America. That gives a sense of the complexity.