That website was set up as a result of the Fair Credit Reporting Act (which also allows people in the US a free credit report once a year) which only applies to Americans.
Consider that they can probably do so simply by using a proxy located in the US, or while visiting the US.
"Legitimate" users don't notice, casually-interested people get annoyed, and fraudsters work around it.
It doesn't solve anything and I wish people would stop doing this.
Basically, credit card companies (and insurance companies) buy consumer data from the credit bureaus to send unsolicited offers. That site is for you to register with the bureaus that you don't want your credit report sold in that way. That's how come companies can still send you unsolicited credit offers when they already have your information from elsewhere, such as from your bank account or frequent flyer account.
I don't know which, if any bureaus operate in Canada or how that are allowed or not allowed to sell your data. That particular site gives you specifics information about American law (FCRA), and how things work in America with credit bureaus information sharing - it's completely irrelevant for Canada.