I don't think it's the fact that anyone expected Winegardner to equal Puzo's writing style for the novel. It's the fact that he felt, for whatever reason, that he could change things that were already established in the movies (which are canon because Mario Puzo assisted Coppola in writing the scripts).

The way that Winegardner wrote GFR would be like someone writing a Star Wars novel and having Emperor Palpatine be Luke's father instead of Darth Vader; it would completely change the storylines that people are familiar with. This is what happened with GFR and, IMO, the reason why we are so vehemently against it.