It seems to me that DC wants to make Superman more of a global figure than purely an American one. I think this is a logical next step for him, to be honest. The idea of globalization has taken hold so firmly now that it was inevitable that a superhero like Superman, who has essentially worked on behalf of a particular govt, now works for the whole globe and will help humanity wherever it is in danger regardless of who it is.
I don't really know what the writers are trying to say by having this split occur in such a manner though. It may simply be some sort of bastardized version of a "statement" about the state and direction of U.S. foreign policy now. As tempting as it is to read further into this about-face of Superman's, I think what's really going on here is that the writers are simply trying to "say something" and they're using Superman and his citizenship as a vehicle to denounce whatever actions of the U.S. govt that may anger them.
I don't think this says as much about the U.S. as it does about the state of comics either. Back during WWII comics were helpful in ramping up patriotism and a lot of the characters (Batman is the first that comes to mind) even urged their readers to buy govt-issued war bonds inside the pages of the comics. Even during the Vietnam War the major comic books didn't play to the anti-war/student protest crowd nearly as much as they played to the fervent patriot crowd. I think comics have always been kind of slow in that regard. There wasn't a real move toward the anti-hero type of character until Batman's renaissance in the mid 70s, the introduction of characters like the Punisher and the overall darkening of the tone of a lot of the comics, even though that movement had taken place in American literature much earlier and in American film back in the late 60s. I think what seems to be a pretty overt way of levying criticism against the U.S. govt and its foreign policy is the next natural step as comics slowly crawl toward being overtly political in an anti-govt sort of way. At least, it's moving toward the mainstream now with a character as intrinsically-connected to America as Superman.