Except that there are several invasion risks, especially in the north. Canada maintains bases there (Alert) to protect its north from being taken by the likes of Russia, the USA and even Denmark (they have a longrunning dispute regarding an island near greenland). Canada also does not want the northwest passage to become an international waterway and so must maintain control over vessels in the north.
https://en.wikipedia.org/wiki/Whisky_War
>> The Canadian government issued a declaration in 1986 reaffirming Canadian rights to the waters. The United States refused to recognize the Canadian claim. In 1988 the governments of Canada and the United States signed an agreement, "Arctic Cooperation," that resolved the practical issue without solving the sovereignty questions. Under the law of the sea, ships engaged in transit passage are not permitted to engage in research. The agreement states that all U.S. Coast Guard and Navy vessels are engaged in research, and so would require permission from the Government of Canada to pass through