I think it has something to do with the fact that the VC is full emulation and the demos are wired as part of a Wii game (not technical enough to, uh, know if that's true.) The fact that VC games appear as if you've plugged an NES or Super NES into your fancy TV is, IMO, a great thing. It means the game looks EXACTLY like it should, and if you did have your NES or Super plugged in you'd have to fiddle with your TV settings to make it right.
Based on the widescreen TVs I've had, most can switch between widescreen and fullscreen modes via the remote, which is way easier than changing the Wii settings.