Its been argued up to high heaven. We all know they don't do it. Only thing I can figure is they're using the Wii to make cheap games (which is also well known) and don't want to spend any money on advertising because they just want the games to make pure profit. As long as they can make a medium quality game for 500,000-2 million and turn around and sell 100,000-300,000 copies without any advertising, they're happy.
On the flip side, if they make an HD console game, which costs way more to develop, they not only want to make sure the game is successful, they HAVE to market it in order to make sure they can make their money back.
Long story short, developers are using the Wii to make money with cheap low quality games they wouldn't put on other consoles.