I am guessing that the reason some people choose to write "USian" instead of "American" is because they worry that if they don't specify the US, natives of other countries in North and South America might think they are being referred to as well? Or do they fear that if they use "American" to refer to US citizens only, other people in the western hemisphere will be offended by the implication that they are not American?
In either case, the distinction seems unnecessary to me. As a Canadian citizen raised in North America, I have never once thought of myself as "American" or resented not being included in that term, and I'm pretty sure most of my fellow Canadians feel the same. In fact, it seems to me that Not Being American is one of the three great Canadian national pastimes, right after hockey and going through the drive-thru at Tim Horton's. So there's no need to use "USian" to refer to the people of the United States, because when we hear "American" that's all we can or want to think of in any case.
But perhaps I'm mistaken about this. So I'm asking my readers who live in North or South American countries that are not the US to tell me how they feel about the matter:
What do you think of when you hear the term "American"? Does it seem like it refers to you? Do you think it should? And what do you think when you see writers using "USian" instead?