There are plenty of family-friendly games that are excellent. World of Goo. Super Swing Golf. Trauma Center (even though it is rated T for some reason). And Nintendo has never been against M-rated games ever since the rating system was introduced (they've published a few, like Perfect Dark, Eternal Darkness, and Geist, and have never put any restrictions on what other developers can put on their system beyond the "No AO games" that Sony and MS also do).

But getting back to the issue you are really addressing, it's just that way too many people out there think game consoles should be aimed at an individual market, not the games. Which is kind of ridiculous, since nobody thinks particular brands of DVD players should only play kids movies. But that is the mindset. They'd never think twice about the DVD player in their home playing a violent movie for their kids without them knowing, but they want every game on the game console they buy their kids to be suitable for them. I guess so they don't have to be actual parents.