Well, I just gave you an example of people from outside the US being referred to as Americans. It's a political issue, though, like the use of "men" to mean "people", so you can't solve it by appealing to common usage. People who don't like your usage are still offended by it even when they know it's common. And they probably won't bother to talk to you, or, in many cases, even learn English.

Me, I'd prefer to be able to listen to them, especially since I live in Buenos Aires. So I'm going to continue to not use "American" in that sense, which means I still don't have a better alternative to the abominable "USian".