Hello hello~ I'm Cielle, the author of Reus. I'm from the US. I'm living in Colorado right now (I live in Boulder), but I've lived all over (born in Texas, school in LA, summers in France and Denmark, a semester in Japan...I get around ^.^) I just say I'm from Colorado, though.
A few people have already weighed in on "why do people from the US list their state and city?" and I think it's been covered pretty well, but let me tell you one sad-yet-illuminating fact about the US: Only 36% of US citizens have a valid passport. Yikes.
A lot of US people feel that there's "no need" to leave the USA. Maybe it's because the US is pretty diverse, geographically and culturally speaking, and staying within US borders means no currency exchange and no language barrier. Some people feel like they've left the country even if they just visit a different part of the country! Just take LA and NYC for example; two of the most internationally known US cities. Angelinos are reaaallllly different from New Yorkers. Moreover, the weather, food, music, geography, etc., in California is vastly different from New York. Even within a state, there are a variety of different cultures. For example, people from eastern Colorado are predominantly farmers and agriculturalists and tend to be more conservative, whereas people who live in "western" Colorado (Boulder/Denver Metro Area) are a mix of physicists, hikers, crunchy granola people, and city folk, all with more liberal inclinations.
also, I don't call US people "Americans" because there are a lot of other countries in the Americas besides the US. Why should one country get to claim the "American" title? Anywho. Sorry this was so long haha. It got out of hand quickly~