Yes, the border is open to commercial traffic, AND US residents passing thru to get to Alaska. They must drive directly through Canada, the shortest route possible. Unfortunately, some of those people take detours. They've been found on Vancouver Island, Banff and Vancouver just sight-seeing. I have zero sympathy for these people. Matter of fact I'd like to see them not only fined, but their vehicle impounded for 14 days before they can get it back.
Now, I assume that's not quite what you are talking about, you are referring to Canadians with US plates because they work and reside down there and have come home. Some don't change their plates because of costs and/or they never expected it to last this long and there have been lots of incidents of "plate shaming".
Now I feel for these people, but if I were in their shoes I'd be doing everything to make it obvious that I fall into the category of "Canadian but with US plates". I've seen cars with signs/stickers on the back window or bumper that say "HEY, I'm a Canadian with US plates" or something to that effect. Just makes sense to do that in these times.
No offense, but these days with Covid and the USAs vapid response to the pandemic, I don't want visitors from the states coming up here and potentially ruining our efforts to flatten the curve. Unfortunately, there are a few people who fall victim to the more overzealous of us up here who feel the same way I do.
Keep in mind that this is happening up here in Canada where we are generally known for being nice and polite, something you folk down in the US often make fun of in safer times. I can guarantee that if the situation was reversed and we were bringing such risk across the border into the US we would receive a rougher reception.
Sorry, just reread your post and I apologize if my tone was a bit confrontational. Just a sign of the times we live in these days.