Since Nintendo is a Japanese owned company it's still Nintendo. Doesn't matter if there is a Nintendo of America, thats just one part of the company. But the Japanese are the head of it.
So it doesn't matter if Nintendo of America wants it. Japan is where it makes most of the money to so they will meet the demands of the Japanese the most. Plus alot of casual old farts are buying the Wii's up so Nintendo is still doing good here without online. So if they are making good money here with the casuals and Japanese think online is "nerdy" there's no point in there eyes to do it.
Which is why I really like 360 being a American company because they look to the needs of Americans first, while the Japanese suffer.