I think the reason is basically due to the games themselves already being region locked. It would be like having different games for different parts of the world, but then allowing for international demos. It makes no sense. Streetpass, on the other hand, is a system-level feature and is identical throughout the world.
Until you've played it, every game is a system seller!
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mini-games on consoles, cinematic games on handhelds, what's next? GameBoy IMAX?
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