The hysterical kerfuffle over the latest issue of Action Comics, in which Superman renounces his American citizenship, has predictably inspired a huge amount of jingoistic blathering. And of course there have been plenty of calm, well-reasoned rebuttals from people who actually understand the point of the story and who aren't, y'know, morons.
But one counter-argument I've heard around the Twittersphere - partly in jest, partly not - basically amounts to, "Calm down, Superman's not real." There are many valid reasons to reject the anti-Superman screeds, but this isn't one of them. If you believe that fiction has a serious role to play in demonstrating and shaping societal values, then it's perfectly reasonable to be angered by a work of fiction that seems to spit in the face of the values you cherish - or worse, one you once regarded as sharing your values which suddenly seems to upend or reject them.
This is doubly true of an iconic character like Superman, who is as potent and lasting a mythological figure as American culture has ever produced. More than any other superhero - probably more than any fictional character we've birthed - he has been a model for the better angels of human nature. He's lasted as more than a kid's comic book figure because of the high ideals he represents. Existing only in ink and paper doesn't diminish the real-world importance of those ideals; and that means any complication, alteration, or subversion of those values (all of which are inherently subjective) is fair game for an honest emotional response.
To put it another way, remove the context of the citizenship issue. Suppose DC released an issue of Action Comics in which Supes straight-up murders a bunch of bad guys - chucks them off buildings, mows them down with his heat vision, what have you. Intelligent, rational readers everywhere would be fucking furious, and rightly so - it would be an utter obliteration of one of the character's most fundamental values: unyielding respect for human life. That none of those deaths really happened would be entirely beside the point.
Basically, you can't have it both ways: If we can be inspired by a character's positive attributes, we can be incensed by his negative ones. Either you believe fictional actions have a real meaning, or you don't.