They allow Canadian plated cars one-way to US border cities. But they require US plates for one-ways deeper into the US. I'm curious what would have happened if that's what you had wanted to do. It's their own fault they forgot to hold a US car for you.
As an aside, it can be difficult to adjust the dashboard information display, even for US cars. 2 weeks ago, I picked up a Chevrolet Sonic sedan from National Seattle Downtown, one-way to BOS. The car had NV plates, and the radio clock format was Pacific military time. I thought about correcting it as I crossed the time zones, but wasn't sure how, and didn't want to waste time fiddling and possibly breaking it. I figured let it be Logan's problem to change it for the next local customer. So the clock was still on PDT when I returned it there. The check-in guy didn't say anything about it, and I didn't either.