I don't think it's about Americans buying gas per se. I think it's about Americans having infinitely more than they need. Don't know that I'd go as far as saying that it directly supports terrorism. However it does contribute to how the world views us.
Honesly, how many of us really need an SUV? Gimme a break. I'd like to see a study done that answers questions like:
When was the last time you put that thing in 4wd?
How many days out of the year do you drive that thing to work alone?
We are Americans. We are gluttons. Right or wrong, we should at least admit it.