I think that people merely want to shorten the name of where they're from. If someone is from the United States of America, it is much easier to just say America. If someone is from South America, or Canada, they are not going to say, I'm from North America, I'm from Canada... they are going to say, I'm from Brazil, Argentina, or Canada. What makes the most sense for people from the USA to say is, I'm from America. Everyone knows what they're talking about and I don't see a problem with it. America is the name of the country, in a shortened form. That's it.