How do they do that? I've driven from the US to Canada dozens of times, and at every port I've been to the road goes directly past US customs up to Canadian customs. US customs has no opportunity to stop me coming into Canada, unless there are CBT officers in the Canadian customs booth?
You may not interact with them usually but that doesn't mean they can't stop you. They do it before you pass them. I've had it happen in both the US and Canada.
Yes they can stop you - but to say it's routine sounds wrong - I will admit I haven't driven across the border in a long time, but when you head to the US, you don't normally deal with canadian border authorities, and vice-versa - you juts deal with the authorities of the country you are entering, not the one you are leaving.
