In some Canadian airports, there's "pre-clearance", where you go through American customs while still on Canadian soil. Right now, they can't hold you, because you're in Canada -- so if they aren't going to let you in, you can just leave, back into Canada.

There was a recent (Canadian) bill that gives US customs the ability to hold and search in pre-clearance, in Canada.

If the alternative is just to go back to Canada, why would they want to hold you anyway?

Let's say during the search they find a conspiracy to commit a crime in the US. They want to hold you, extradite you to the US, and throw you in jail.

Wait. In that case, wouldn't the crime (conspiracy) have taken place in Canada? There should be no extradition, then.

It's the pre-screen areas are inside Canadian airports, on Canadian lands. Anything the us border cops can do there, they do so with the permission of the Canadian government.

Which really, should be whatever they need to do. The twist being at any point you should say "No, I'm staying in Canada" and they should say okay bye.

I think that happened in the article.