Why "Press A + B" to start a Wii Game?

Forgive me if I’m being obtuse, but I have no idea why Nintendo forces gamers to “Press A + B” to start a game on Wii. It was like this when I played the system at E3 earlier this year, and every trailer showing a start screen since then displays the same. Why two buttons to start? Why not just “Press A”? Don’t get me wrong, pressing two buttons at once is no big deal, but the usability ninja in me just doesn’t like it.