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.