What matters is that Superman is an American Icon and should be portrayed by an American. It's not about the acting, it's about the nature of this one character in particular. I don't have a problem with it when it's Batman or Spider-Man being played by Brits, because those characters aren't explicitly stated to stand for 'The American Way' and aren't constantly posed with the Stars and Stripes.
Superman is a special case.
That's ridiculous. I don't see why anyone would mix the CHARACTER and the ACTOR
portraying him up outside of the movie.
It doesn't matter what Superman stands for. That wouldn't change just because some British guy is playing him. It's what actors do.
And realistically, the average joe probably wouldn't even know he's not American until they heard him doing an interview or something, assuming he can do a good American accent for the movie.