No-brainer? Really?
Have you seen any pictures or video of ANYONE playing a Wii game left-handed? In fact, has there ever been a left-handed controller (save for the goofy one-handed PS1 controller)?
Giving someone the option of playing left or right handed would nearly DOUBLE the amount of QA time it would take to test the game, since you now need to go through everything twice, once in each configuration. Why? Because you have the potential of one small change cascading through all the code, and causing a bug that just doesn't happen playing right-handed. I can't see anyone spending the time and money to both implement and test such a feature, since we've all been conditioned to play games right-handed anyway.
Of course, the one exception to this is the Wii-Sports games, or other small mini-games, where the code is simpler, and the gameplay naturally lends itself to "handedness".