I believe these are people who have applied for refugee status in the US but for whatever reason have been denied.. then I think its ok to try and claim in Canada.
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curiousgeorge curiously does not know refugee law

nor the treaty between USA and Canada about third party safe country
Both Canada and USA are "safe" and do not take refugees from each other
Refugees apply in the first safe country they land in

Smug Canadians will argue that USA is not safe
now we are not discussing refugee law
but delusional moral superiority