Why some fans enraged of Goku's ethnicity?

I have no idea why some DBZ fans enraged when Goku in Dragonball Evolution was portrayed by a Caucasian actor. They said it will be better if it was portrayed by an Asian one like Chinese, Korean and Japanese but I just don't care. As far as I know Goku is an alien from planet Saiya and aliens are free to be portrayed by anyone else. Don't get me wrong I'm an Asian too. I'm just don't get why some fans can get enraged by it unless Dragonball Evolution is a bad movie.