What makes a film American?

So the BBC asked critics around the world to choose their top 100 American films. I couldn't help noticing the rules would exclude Let Me In. The nationality of the director means nothing (Hitchcock got several films on the list), the setting means nothing. Casablanca landed at #9 with a Hungarian director...and obviously set in Casablanca. As always, it's about the studio that makes a movie.

What defines an American film? For the purposes of this poll, it is any movie that received funding from a US source. The directors of these films did not have to be born in the United States – in fact, 32 films on the list were directed by film-makers born elsewhere – nor did the films even have to be shot in the US.