Incredibly, people in USA are so self centered and egotistical that they forget that north America and south America and central America exists, and not only that, they seem to forget that we are all Americans. So this poll title is not only inexact, it's also showcasing a level of hypocrisy and self entitlement that was never thought to be possible.
What's your alternative? Other countries have similar one-word names for their citizens, "Italians," "Japanese," "Mexicans", etc. United-Statesians? Then we might get the complaint that many other countries are made of smaller states too! Let's go for "citizens of the United States of America," that's nice and short. I get that Central/South America exists, but let us have the word, there's nothing else which sounds good.