Paul - Very similar to the age old question, "Which came first, the chicken or the egg?" :-)
The fact of the matter is that the indigenous people are not equipped to deal with these types of food, but neither are we! I think what's really atttention grabbing about groups like the Maori and disease is the stark contrast before and after the introduction of modern food. No one is going to take notice when someone in the western world is fat or diabetic, but when you have an extremely healthy people who start to resemble a western culture (in terms of health) it's like night and day.
Back to the diabetes and fat issue. I don't think that it's necessarily one way or the other. People become fat and/or diabetic due to insulin resistance. IMHO, testing for insulin levels would provide a much better picture of the health of a person. You can have normal sugar but abnormally high insulin levels. As you become more and more insulin resistant, your body has to pump out exponentially more insulin to keep your sugar stable. What I'm getting at is being thin does does not mean you can not get diabetes (without gaining much weight) and being overweight does not necessarily mean you have diabetes. The overweight condition and diabetes both stem from the same pathology, but they are not always intimately connected.