The difference is once a game is released for Wii, Nintendo can't turn around and say 'You can't sell this any more.' Once you're approved, you're approved, and you can go on to profit from your work. They have known rules for what kind of content is allowed that don't undergo major changes every couple of months.
By contrast, Apple could decide tomorrow that all apps with purple backgrounds aren't allowed, and pull them all from the store immediately without notice. Then the next day they can decide that purple is okay, but only if it's darker than a certain shade. Then they could drop the whole rule again the next week. They reserve the right to change the rules at any time for any reason.
How many wiiware/xbla/psn games have been pulled after release due to changing rules about what's ok and what isn't?