What are the credit scenes supposed to mean?

Yes, but you know that he can't mean that, so you are here looking for a discussion. Bravo :D

Here are some other things I think about when I watch this part:

-- Americans are considered very naive and idealistic people as a nation. After all our history is only a somewhat more than 2 centuries old. We run around policing the world and passing judgment on other peoples, many of whom have cultures older than ours, and yet we have so many problems in our own country that we have no good solutions for - poverty, drug-addiction, racial tensions, obesity, lack of universal healthcare, hunger, violence. We are preaching democracy and capitalism, but are these systems creating the best society, if we can not address our own failures?

-- Americans have primarily two parties, and they often split along the lines of what Grace and her father argue over. Either you accept the weakness of human beings and try to support them through their trials with socialistic efforts like welfare, unemployment benefits, disability, and such, or you insist that each person carry his burdens no matter how heavy, creating a society where a higher percentage of people are imprisoned for crimes than in any other nation in the world. See statistics at http://www.nationmaster.com/graph/cri_pri_per_cap-crime-prisoners-per-capita Have either of these approaches solved our problems? Is there any other way?

-- Americans have both power and responsibility to the rest of the world, as we have vast resources and wealth which have made us a superpower for decades. We should try to use our abilities to make the world a little better, and yet there is no consensus as to how this can be done or what is really better. Let's do better than what has been shown to not work in this allegory. Learn our lessons from thoughtful contemplation and discussion of interesting stories like this one told through film, rather than through reactivity to horrors and tragedies like 9-11. Look at how we have massively overreacted to 9-11, leaving the rest of the world aghast at our warmongering and the internal terrors of security checks and privacy invasion and the use of torture and the utter failure of our moral code.

-- Look at the people and see the priorities of the poor and wretched. They have chosen to purchase and keep guns, when they have no sheets or bed for their mattress. They are buying booze and smokes when they are emaciated from malnutrition. Why do they do this? Who is to blame? What can be done? It takes both good givers and good receivers for real change to happen. We need to be careful and keep trying. Try, try again without losing ourselves. We can not walk the path of Grace, nor be gangster, nor be dogs. We have to be strong, give and take with love without hurting each other.

The credit shows that people, even the most simple and possibly loving like the ones in the photos, have the capacity to be cruel and manipulative even if the exterior appearance shows otherwise

I suspect to show a "reality" of America that is contradictory of the image of America presented to the world through conventional media. As Europeans, we're bombarded with American culture, or at least one perception of it. This perception tends to be of wealth, beauty, sophistication and fun.

Even films and sitcoms that present "blue collar" characters have them living in big houses, going on adventurers they could never afford, have them looking physically perfect at all times, show them to make amusing interactions, etc. The American culture presented in this media mocks the appearance of other countries; the French are dirty and smell, the Brits have bad teeth and rotten food, the Irish are drunks and backwards, the Germans are emotionally detached fascists, the Dutch are pot smoking pornographers, the Swedes are depressive minimalists, etc. They revel in these stereotypes but rarely make light of their own.

The closing credits of the film seem to exist to rub the collective noses of this American culture in their own squalor, their own misery, their ugliness, their own problems. It's an ironic counter-point to what we've seen in the film and an inherent part of the films political subtext; that America should concentrate on sorting out its own innumerable problems instead turning its attentions elsewhere.

The juxtaposition between Bowie's song (another outsider's view of American culture, both critical and celebratory) and the brutality of the imagery is of course a provocation, and as ever with von Trier, a "bad taste" one, but it seems essential to underline the idea of American hypocrisy as suggested by the narrative itself.

I don't think that is the case OP, in fact, I think to some extent it is closer to the opposite:

Many critics have treated it as a flippant piece of Anti-American sentiment, which is mostly wrong, but not entirely. After Trier made Dancer in the Dark he was heavily criticized by some, along the lines of 'who are you to make a film about America, you've never even been there'. I would suggest that Trier, having a fairly dark humor and a strong sense of irony, decided that this credit sequence would be a nice little 'FU' to such critics. Certainly, Trier knows that it is likely to inflame.

However, there are also some arguments that instead concern the substance of the film. Arguably the credits are something of a reality check. Trier knows his audience will, generally, relish watching this awful town get its comeuppance, and the 'Young Americans' imagery can thus be seen as a reminder of who these people really are - the bottom of the barrel, the poor, the mistreated, the uneducated etc. Should we really be taking the stance that Grace's father (The Big Man - theological reference to God) advocates, i.e. 'arrogance'-free Old Testament justice? It is worth noting that although this 2nd interpretation would be quite a reversal, Trier is like that - recall the other one that occurs when Grace goes from sacrificee to sacrificer once her gangster father & associates (a deus ex machina of extreme proportions) arrive.

Personally, I think that the first view is hard to deny, the second view is also apparent. This is particularly so on subsequent viewings, although the later 'significance' is undoubtedly more open to interpretation than the first.

Is this your homework, Larry?