The mission of protecting North American airspace is a joint undertaking of Canada and the United States.
A decision to shoot down an aircraft over Canada is a Canadian one. When we practise Operation Noble Eagle, the authorities to act, even if it's an American aircraft coming across the Canadian border, remain a Canadian political decision to take on that shoot-down, and the Americans would not unilaterally take a decision unless they thought that there was a threat to the United States.