Of course they love party games in Japan.
Have you ever been to Tokyo? You can't sneeze in public without giving the flu to at least a hundred "gamers."
There's almost no point to online play in Japan. You're never for a shortage of immediately available participants in almost any form of digital entertainment.
If you could walk into any subway car in Manhattan and complete a NCAA tourney sized bracket for a Mario Kart DS ad-hoc Wifi competition before getting to your next stop, then I suppose it makes sense that you wouldn't care about the whole online experience.
I have exactly five friends who play any kind of video games, and not a one of them lives anywhere near me, and every one of us owns a DS and a Wii because we like the Nintendo franchises, and because with built in WiFi we just assumed they'd have... you know... some decent online play.
That's what we get for being stupid Americans I suppose