It is a treaty to which Canada and the U.S. are signatories. I'm not an expert on international law, but I know from my perfunctory reading of the Chicago Convention of 1944 that 52 states plus Canada signed a treaty that's still in force today, which states that each country maintains sovereignty over its individual airspace but determines who can enter its airspace.
I know we have only a couple of minutes left, and we're not going to be able to get into a full international law debate, but you'll certainly agree with me that the ability to sign binding treaties is an aspect of the sovereignty of a nation.