Edit: Actually, when is there ever any confusion? There isn't a whole lot the "Americas" as a whole have in common. If you want to do continent-based demonyms, just prefix North or South.
I was initially being sarcastic but if you want to get serious on the topic, let's at least admit that there would indeed be confusion for an extraterrestrial coming to Earth for the first time. Wouldn't you find it funny if people from China were called Asians instead of Chinese or if Mexican people called themselves Americans instead of Mexicans?
If that's the primary confusion scenario, I think I'm comfortable with the way things worked out. ;) Also, I was mostly commenting on wyclif, I got your sarcasm.