Coppola's arm was twisted by Paramount to make The Godfather III, and yes, it was made almost completely for financial reasons, but Coppola still put every ounce of himself into the film. He and Puzo wrote the Corleones in the direction that they wanted. If they were going to write the movie just to be assured a big income, they would have had a lot more sex and violence.
Same with the previous two. Coppola never conformed to "what America wants". He always did what he felt was suitable. He is only mad, and rightfully so, that this movie is being taken to a place that Coppola, the living creative genious of The Godfather (the dead one beiong Puzo), doesn't want.
While I will be buying the video game, and am excited about it, I do feel for Coppola, and understand his anger. I mean, I don't know if they tried, but it would have been great to have Coppola as a consultant on the game, rather then Winegardner.