Why is Gunter Grass so controversial?

Günter Grass is in a very unusual position. It's a position which people would like to take away from him, but can't. You can't take away his company car. You can't deny him a bonus. You can't kick him out of office. He's a self-made man. Now people always say that he should give the Nobel Prize back, since that's the only thing he could do. Or renounce his PEN membership. Grass is a rare example of a fully independent and yet prominent man. And he only has himself to thank for that - he doesn't need to act with regards to others. That's why he provokes controversy.