Let me start by saying that the problem with the Major being portrayed by Scarlet Johansson is not that the Major is Japanese character and can thus only be played by a Japanese actor. The Major's body is pretty much entirely artificial with the exception of her brain and she occasionally switches up bodies so that she looks different (which is a built in escape for changing leading ladies, really). There is plenty of reason for why you could explain away why she looks Caucasian at the moment.
That may make it sound like I am in favor of a white Motoko Kusanagi. I am not. While it is easily possible to explain why it makes sense that she could appear Caucasian if she wanted to, there is also no reason to force making that explanation. If the studio had several actresses of a large variety of ethnic groups, mostly Asian, trying for the role and out of the entire group found that the person best suited to capture the personality of the Major (something I doubt Johansson will pull off) was a non-Asian, then I could see defending the choice. Especially if it was an exception to a normal rule, which this is not. There is a persistence of Hollywood casting white actors regardless of whether the original character is white or not.
