Why would you expect them to, when statistically Americans are more likely to be murdered, assaulted and insulted by other Americans than they are by people of other nationalities?
When there's so much statistical evidence for Americans hating Americans and American-on-American violence, it's no surprise that Americans don't support other American fighters (who are more likely to be the cunt that cut them off in traffic than their foreign opponent).
Of course it's not just Americans, the same can be said of any country where you're more likely to come into conflict with someone of your own nationality than you are a foreigner, so it doesn't make sense that you would automatically root for your own countryman over another person when your own countryman is someone you're more likely to have issues with outside the sporting event.