Franklin Richards hit it right on the head. Superman is an icon meant specifically to appeal to and inspire pride in the American people and our way of life. And while snarky cynicism and self-loathing have become the order of the day, the fact that the ideals embodied in the Superman persona are still attractive enough for other cultures to try and lay claim to him as well goes to show how desirable 'The American Way' itself can be, even in times when our government doesn't live by it.
There's the idea of the American melting-pot, as Kal came to the country as a foreigner in its truest sense. And while he upholds and shows pride in his alien heritage, there's a reason he's not fighting for Truth, Justice, and the Kryptonian Way.
There's the reverence to the traditional honest working family, as he was raised not by some wealthy socialites or political activists, but a kindly farmer and his wife. They instilled in him the courage and strong moral compass that would allow him to make even the most difficult choices. And even in adulthood and at the height of his powers, he chooses to work for a living at a blue-collar job.
Most of all, there's the idea that the individual life is sacred and meaningful, the individual mind is worth fighting for, and the individual man--not the government, not the upper-crust elites, not big business, not the collective, but the individual free man with a good idea and the backbone to see it through--can change the world for the better. Even when Superman has the power to break all the world's armies and set himself up as dictator, he instead labors against tyranny and acts as a servant to the people he protects.
Those are the ideals that, while they had been flirted with during periods like the Enlightenment in Europe, were first truly pioneered and brought into reality in America. And while politicians and semanticists can squabble forever over whether the Left or the Right is out to destroy or restore said values, our people as a whole have every right to be proud of ourselves for being the ones who lit the flame. To that end, we have every right to consider the Superman character a personification of Americana.