Western games have done considerably better on the 360 than the Ps3 or Wii in Japan. Looking at this week as a premier example:

http://vgchartz.com/japweekly.php

We can see that the 360 performs profoundly better than the PS3 or Wii when one considers install base (there are numerous examples of this, and another recent one was the third installment of a Tennis game, that debuted at 2500 for the 360 and 1500ish for the Wii. I can't find it, because it was far down the chart and was 2-3 weeks ago).

My point is that the 360 is heavily tied to Western games there, and it's pretty clear that people who buy a 360 buy it if they happen to be Japanese but prefer Western games (of which there are apparently few). However, if Western games ever DO take off in Japan, Microsoft will be in the premier position to capitalize. Not saying that will ever necessarily happen, just saying that there are upsides to Microsoft's current position in Japan.  

it's the americans living in japan that buy the 360, mostly. Which is partly why COD4 sold better on 360 than PS3.

Call of Duty 4
sell on X360 more than PS3 according to VGCharts

If for some reason the Japanese suddenly stop having their own culture and move on to a western culture, Xbox360 would be (more) popular. That is really not a statement. Here I can do the same.
If the people in the US started acting more like the Japanese the Xbox360 would fail there and Microsoft would lose.

I wouldn't be suprised if there was some of those call of duty games were being sent to Korea

Why wouldn't Korea buy thier own COD4 games?

Either way, it's interesting the Japanese userbase prefers X360's western games over the PS3.

On another note: it also seems that the Japanese are adopting Western games more readily than ever. It's increasing at a snails pace, but it's there. COD4 outsold COD2 and COD3's LTD sales in just 1 week. Great.

Don't forget COD2 and COD3 are about WWII while COD4 takes place during a modern fictional conflict. Maybe I'm wrong but I think it matters.