Actually, in Italy when he was playing at Parma, they called him America or the American.
He in fact he has said he's an American in all walks of life, except when he plays soccer. Which really makes no sense. If he had just come out and said "I'd just rather play for Italy" it would be so much easier than to say what he said.
Look at guys like Subotic and Ibisevic. Both grew up in the US and played footie in the US, college level and US level, both chose to play for different nations. Neither of them are considered traitors because they don't also claim to be Americans. Rossi does, he's trying to have it both ways. I think that's what makes people angriest.