Hey, nice way to sideline the tired old "Nintendo is for Kiddies" argument into this... but all three have announced they don't allow AO games. That's Nintendo AND Sony AND Microsoft. Notice how none of them said word one about Manhunt 2 UNTIL the ESRB gave it the AO. Had it been given an M, Nintendo and Sony would have happily released it. Don't try to see more into Nintendo's marketing strategy than there is. There is plenty of violence available on Nintendo systems. When Mom goes to the store to buy a Wii game, she has to look right at Call of Duty and the Godfather to get to Mario Party 8 and Surf's Up. You're only correct in your point to the extent where it covers Nintendo's already-established family-friendly brands. They protect Mario, but if Nintendo keeps up their momentum and becomes the top dog in the new-gen, you can bet that Rockstar would do a GTA game for Wii and Nintendo would absolutely let them do it. Because everybody likes money.
The real issue is that the ESRB says that Manhunt 2 went over the M-rated line (however "correct" that assertion may be) and that means no retailer wants to sell it. We need to confront society's perceptions of the kinds of content video games can explore, and the opinions of ESRB, Wal-Mart, Sony, Nintendo etc will follow. These companies don't give a shit about morals, they care about not getting dragged down into bad press and Moral Majority fights.