I think the best way to answer that is to simply state that Canada and the United States, through the treaty, have agreed that it's best if people seek refuge and explain why they need protection in the first available country, as long as that country has a robust asylum system, is a safe place, and so on. We have mechanisms to verify that against each country, and we've done that, to keep the agreement current.
Then you get into the second area of your questions, which is the concept of turning back. Canada would be contravening its obligations on the international scene with conventions against torture and refugee convention by turning someone back who sticks up their hand when they cross the border and says they're in need of protection.
This is a two-part response, but those are the main fundamental areas around this concept of turning back.