The point isn't the whiteness of the singers, its the whiteness of the fans.
It's worth pointing out that Al did this polka around the same time that he worked with Ben Folds on "Rockin' The Suburbs". Indeed, his own Ben Folds-assisted style parody is three songs later in the album.
Paul McCartney disapproved of Weird Al's "Live and Let Die" spoof "Chicken Pot Pie" because... Paul is a vegetarian? What sense does that make? Weird Al is a vegetarian too!
For one, Al wasn't yet a vegetarian when he first wrote the song. Also, Paul (well-known for his social crusades) is one of those vegetarians. In fact, I'm pretty sure he went full-on vegan after Linda died.