We base this on various information, including source of funds and IP address. If you used Dwolla (which is only available to US customers) or if your bank reported you as a US citizen when sending funds to our bank, then you are automatically considered a US citizen.
Given your talk of citizenship rather than residence, would I be right in assuming FATCA was an element in this decision?
It sounds like US citizens who live outside the USA will not be able to trade on Mt Gox at all. Indeed I suspect no exchange that's sensible will take them
I am not such a person fortunately, but I feel sad for those who are.
That's what I think too, but it seems MtGox will also apply the same restrictions to Canadians, so I assume that Canadian customers were sold to Vess as a package deal.
Both Canada and Japan have way stricter privacy laws than the United States so I would not be surprised if a Japanese company denying service to Canadian customers because the latter choose not to waive their privacy rights to a level below that of both Canada and Japan is against the law in both Canada and Japan.
Treating Canada as the 51st state of the United States can open a huge legal can of worms here.