Well I believe that it stands for American but come on that isn't that hard to spell. 
To us in Europe, people in Canada, Mexico, Argentina, etc, in fact every country in North, Central and South America are "Americans" really. So USian makes clear where you mean. (Of course everybody knows what "American" usually means) Also writing "USian" is a sort of way of letting people know that you don't necessarily think the sun shines out of Uncle Sam's *censored*, that grew up during Dubya's reign. In France, my other country, we call them "Ricains".
