Yes AC is within its rights as a non US based company to do whatever it is allowed to in Canada for flights not touching down in the US. However, one of the requirements to operate flights in and out of the US for any airline is it must adhere to DOT and other US air regulations. DOT can cancel their landing rights. Does this matter to AC right now? Apparently not if they refuse to refund cancelled flights to and from the US. Maybe they are OK with losing their flights to the US.
Credit card chargebacks are also difficult in this situation. You have received a voucher good for the amount of the flight if the flight is cancelled. So technically you have received something more than nothing that has value. Chargebacks work when you received nothing for the purchase price.
As far as where the ticket was purchased, it does not apply here as far as I can understand things. Also, AC maintains a separate web site for the US, so your purchase is made in the US, not Canada, when you use the web page. I just tried to do a booking and the AC Canada web site would not allow me to since my IP address is not within Canada and forced me to the US one. It was quite insistent.