I am not sure I agree. Do you know what I do when I have trouble reading the text in a page. I increase font size. It is only a flick of your finger (ctrl+mouse wheel). So the point is if you have a big enough font size, white on black can be just fine.

Also, should we assume that the people have their vision uncorrected?

I think that black on white is just a fad, perpetuated by following conventions. Like glossy screens.