You mean we have access to their airspace, for our own interests, and Canada’s own interest. I wonder how much Canada is paying the U.S. to offset that huge expenditure.
Imagine if you had to defend US airspace from the obvious polar air route, if you had no access to Canadian airspace. Back in the day, we had the DEW Line - Distant Early Warning - a line of radars in the Canadian arctic to detect bombers, missiles, etc. Also, in the day, nuclear air to air missiles were contemplated to break up Russian bomber attacks. Better those nucs went off over Canada than the US. The attacks never came, but you can argue the defense was effective. If we were forced to defend the US only at the US borders, it would be a much easier task to attack us.
Canadian defense is really an afterthought.