As you all surely know already, Guitar Hero 5 contains a likeness of Kurt Cobain who "plays" two Nirvana songs. But that's not all you can do with Kurt's character in the game, and that's what Grohl and Novoselic have a problem with. Here's what they had to say about it, verbatim:
We want people to know that we are dismayed and very disappointed in the way a facsimile of Kurt is used in the Guitar Hero game. The name and likeness of Kurt Cobain are the sole property of his estate - we have no control whatsoever in that area.
While we were aware of Kurt's image being used with two Nirvana songs, we didn't know players have the ability to unlock the character. This feature allows the character to be used with any kind of song the player wants. We urge Activision to do the right thing in "re-locking" Kurt's character so that this won't continue in the future.
It's hard to watch an image of Kurt pantomiming other artists' music alongside cartoon characters. Kurt Cobain wrote songs that hold a lot of meaning to people all over the world. We feel he deserves better.