Canada also serves as a gateway for illegal immigrants who are not Canadian citizens. That includes many Mexicans, oddly enough. But also many Chinese and Middle Easterners find their way in by way of Canada. It also includes many Canadian citizens who are themselves immigrants or descended from immigrants. And legals who are not citizens.
Remember that Haitian trying to sneak in inside and an athletic bag?
http://www.saveourstate.info/showthread.php?t=1331
Our borders are very porous and we have many authorities in Canada who are as hostile to US sovereignty as leadership in Mexico. We need a national policy on all fronts and not just measured by Mexico.
Maybe the main difference between Mexico and Canada is that Canada is not as corrupt and Mexico serves as a gateway for so much drug trafficking. So much of it is grown in other South and Central American countries. Latin America pretty much owns coca production.
Of course a certain amount of marijuana comes in from being grown in Canada, but the weather is not nearly as good for it as it is in Mexico.