It was justified as any arrest is -- people are generally arrested and then taken to court to have their outcome determined, not the other way around.

Your question phrased another way:

> "Does it imply that anyone wanted by the government can be detained by the police and the government needs to establish the legality of it only after the fact?"

Yes, very much so.

I suspect if the US were randomly and wantonly requesting illegal extraditions to harass people located in Canada that Canada would simply stop making provisional arrests pending formal requests for extradition and require the paperwork up front.