first of all, you're making a generalization that is pretty much ridiculous. Nearly all homebrew apps use the whole screen, so seeing how one app doesn't use the full screen and then thinking it happens on all apps is...well, stupid (if there's more than just wiimc: please tell us).
second: as all decent media players, wiimc uses the original view and scales that to the maximum size. However, as there are two common video formats (16:9 and 4:3), displaying a format that doesn't correspond with the format of your television results in black edges. It's not a bug, and it's no indication that the coders can't make the view fill the entire screen. It's just a silly option to actually stretch it that way, as the result wouldn't look very nice.