I fear worse things happening to them than being just booed at, I hope I'm wrong.
As far as brandishing the flag is concerned, I think the their flag means more to Americans than their national flags do to the rest. Nobody else that I know of pledges allegiance to the flag like they do in the US. I was in the US when 9/11 happened, and I was impressed by the show of flags everywhere, after it happened, and the news that stores had run out of them within days.
Giok