I was specifically discussing international to international. While this was a Eurocharter (NATO) and we sorted out the issues with some pax before heading on to a third country. CBSA was adamant they did not allow it and required docs on everyone. Regardless, whether I enter Canada on my Italian or US passport for int'l to int'l I have always had to process also. That said it seems YMMV, so remove the Canada part of my post, the US part still stands (though I did note 'for the most part' because there are exceptions, even on the US side-as a matter of planning it should not be expected.)