Why not? Denying 41 million a right to vote is a minor crime compared to forcefully conquering a peaceful neighbor that had been your closest neighbor. You're talking about a conflict that would easily kill 10% of the Canadian population, and likely level every major Canadian city, by the time the resistance efforts were finally stomped out.

He's saying they would make a 51st state eventually. The US's client state, Israel, denies the right to vote to nearly half of the population of the areas it controls. And we're their greatest ally. Why can't we deny the right to vote to 10% of the people in the territory we control? (Canada's population would represent about 10% of the combined US-Canadian territory's population.) Hell, we already disenfranchise millions due to felonies. And we disenfranchise millions through voter purges. And it was only in the 1960s that we stopped outright legally disenfranchising people due to skin color. You're seriously trying to argue that a fascist government would have moral qualms about disenfranchising large numbers of people!

The US could quite easily even go far as to say, "all Canadian citizens in the occupied territories are resident aliens and will not have the right to citizenship. Their kids will have citizenship, but no one who has ever held Canadian citizenship will get US voting rights." Every Canadian currently alive simply never gets to vote.

This is entirely possible. A right wing authoritarian government is not stupid. They're not going to immediately grant voting rights to people that will immediately vote them out of office. The only way they would do that is if they were confident that elections were so utterly corrupted that giving Canadian's American citizenship wouldn't change the outcome.