When you have sovereignty, you have an element of control. You act in your own interest. When you don't have sovereignty over certain items, you depend on others.
In the case of the ballistic missile system right now, NORAD is a common-law relationship that is sealed with a NORAD agreement. You're married, and it has obligations in a common-law sense.
USNORTHCOM and CJOC are good neighbours. As good neighbours, you have the choice of whether you're going to help your neighbour or not. The decision to engage, when it comes to Canadian territory, is entirely within the purview of the United States. Their own interests determine whether they're going to do it or not.