No joke! In Action Comics #900 the above panel appears. Apparently the comic book is trying to be morally responsible.
Perhaps you, like me, are thinking maybe he got fed up with the US' imperial designs. Far from it. This action is NOT in response to US policies of aggression and interventionism. It appears that Superman does this in order to act more globally, and to not appear as if acting under the US flag. Its a change of colors basically. From the article
Superman replies that it was foolish to think that his actions would not reflect politically on the American government, and that he therefore plans to renounce his American citizenship at the United Nations the next day -- and to continue working as a superhero from a more global than national perspective. From a "realistic" standpoint it makes sense; it would indeed be impossible for a nigh-omnipotent being ideologically aligned with America to intercede against injustice beyond American borders without creating enormous political fallout for the U.S. government.
Why is this important? Because children read comics. In the absence of a guiding society, children gain their morals from what they are exposed to. Now they're being led to believe that globalization is a good thing. Imagine that, popular comics supporting globalist agenda. Who'd have thought it possible. [end sarcasm]