It's a matter of reasonable effort. How can a company determine that a given email destination is Canadian? It really can't. So Canada's laws are affecting the whole world as companies have to either give up on things that people likely actually want (security bulletins) or scramble to form opt-in databases on worldwide recipients just because of Canada.
Just like many of the laws in the US that people scorn, this Canadian law will only hurt the legitimate people who are trying to be respectful and operate as a good company with records and such. The spammers sending pharma spam and malware spam and such are operating from locations that don't support easy tracking for applying penalties. Thus millions of people worldwide are suddenly getting flooded with requests to keep sending mail (I opted in three years ago!) just in case they might be Canadian.
Therefore the obvious (but depressing) solution is to create borders on the internet and say "To prove you are a Canadian and protected by this Canadian law, you must have a .ca email address. Anybody who does not have a .ca email address cannot bring charges against a company sending email in violation of a Canadian law because they did not identify themselves as Canadian to be protected by the law." This is obviously not-good, but the alternative is a minefield of international laws that strangle the internet and any companies that operate on it.
Fictional but getting less farfetched example: Some Canadian posts a picture of their dog spinning in circles on a video site. The dog is not neutered and there is a flash of anatomy at 1:33 into the video (it's a long video of dog-spinning). Person gets in legal trouble in some country that: 1: Holds content posters liable for their posts. 2: Enacts a law that prohibits the depiction of any sexual anatomy online for the protection of the children/morality/whatever. Suddenly Canadian is subject to fines/imprisonment/death-for-insults-against-the-god because of this?
It seems like a ridiculous example now, but with the slippery slope we are heading down, it's becoming more and more possible.