Oxymoron of American Culture?

I haven't done much research on this, but one of the common accusations leveled at Americans is that "Americans don't have any culture." Being an American I find myself called upon to argue, although usually I just smile and let the conversation go on to other subjects. But what does having culture mean, anyway?

Does it mean having art, great works of literature and a thriving theatre scene? If so, America has those things: distinctive art styles, authors such as Hemingway and Vonnegut, and the repertory theatre movement has maintained theatre as a thriving means of expression throughout the country.

Does it mean having specific traditions, events, dances and music? American holidays are rather uniquely celebrated, they're full of commercialism, yes, but the food and activities are traditional. We've been exporting new music and dance styles since at least the early twentieth century (swing, ragtime, jazz, blues and later rock and roll, rap, R&B) each with specific dances.

What about the other less artistic forms of culture? What about American movies, television and magazines? These aren't too far from artistic pursuits. What about lifestyle? You can't convince me McDonalds and suburbia isn't some form of culture, if not necessarily admirable. What about the internet and mobile phones?

Sometimes the response implies that there is no unique American culture. This has two aspects:

Does the world adopting certain aspects of American culture reduce it's cultural significance? I think so, or at least the apparent magnitude, but for the sake of argument, let's look at other questions too.

Does the diversity of cultures inside America counteract each other so that there is no one 'American' culture but instead African American culture, American South culture, New York City culture, etc?

So if none of these definitions fit, what is it that we don't have that so many people in the rest of the world considers the US to have no culture?

The conclusion I am coming upon is that what they really mean is that we have no history, but I hope to hear some other ruminations on this subject.