Most people think of their location in terms of political entities rather than geography.
there are examples of the opposite. people in canary islands, for example, refer to themselves as "canarios", not "spanish". that makes sense, however, because the term isn't inaccurate.
Point taken, but in that case, their term is more specific and helps distinguish them from a larger group -- the exact opposite of the way you want to use "Americans." And you'll note that they do not call themselves "Africans" despite their islands being geographically part of that continent.
That's not just an American thing.
can you remember any other instance where the name of a whole continent is appropriated by a single country as "nationality"? thought so.
The Republic of India, generally called "India" and whose citizens are generally called "Indians", despite sharing the subcontinent of India with Pakistan, Bangladesh, and Sri Lanka. Do you call the citizens of the Republic of India "RoIans"?
Besides which, the idea that it's an "appropriation" is silly. Next to no one uses it that way in everyday conversation or refers to themselves primarily that way, because it doesn't tell you anything useful about them. If I tell you someone is an American in the nationality sense, that lets you make some generalizations about them -- you know he probably speaks English, you know to a degree what kind of food he eats, what TV shows he watches, who the leader of his country is, etc. (Feel free to snark if you wish here). If I use it in the geographic sense, it tells me... what, exactly, besides the tautology that they live on either the North American or South American continent? That they set their clocks between UTC-3 and UTC-11? Heck, just telling me if a person lives north or south of the equator tells me more about where they live than saying that they live in the Americas.
In short, when a word has multiple meanings, people naturally gravitate towards the meaning that's most useful. Referring to someone as an American in the sense you want to use it is only slightly more useful than telling me they're a Terran.
you should be able to understand how nonsensical this looks from anywhere outside united states.
i know that this is customary in the states, but you guys should also note that this is the internets where you are being read.
It's used that way in plenty of other countries, including Canadians, who would have as much right to be annoyed by the supposed "appropriation" as anyone else. My impression is that they're proud of their demonym and have no great yearning to lump themselves in with a couple dozen other countries by calling themselves "Americans."
Furthermore, at a guess I'd expect that "American" in the nationality sense came into common man-on-the-street usage (as opposed to political tracts or whatnot) outside the US before it did so inside it, because for the first 75 years -- prior to the US Civil War -- most people here tended to designate themselves and others by their state rather than national identity. They didn't primarily think and speak of themselves as Americans but as Virginians, Vermonters, Ohioans, Kentuckians, etc.