Sure, lots of people will get rounded up, but how many will slip the net? How many hundreds of thousands of partisans are spread over tens of millions of square kilometers of rural Canada? How thin can US patrols and round-up efforts spread themselves before they become convenient sources of weapons and ammunition as they are set upon by IEDs and household drones with weapons taped to them? What does it help the US if they know the porn preferences of the guy living in the woods sniping US servicemen?

And then there's the American partisans who understand this is the best time to rebel they are ever going to get. With so many loyalists deployed to Canada, are there still enough to respond to riots, sieges of police buildings, prison breaks, acts of sabotage, etc. in the US? Maybe they can be tracked (or maybe the cameras will be destroyed en masse), but are there enough boots on the ground to stop them?

But the opposite is also true, and it's much easier for Canadians to counter attack on US soil than for anyone else.