Speaking as a Canadian: no "actual, otherwise normal Canadians" 'get "pissed off" when Americans are referred to as "Americans"'. What else are you going to call them, after all? Any Canadian who professes outrage at such is certainly not "normal", and may not be "actual".

Some of us get slightly disgruntled when USA is referred to as "America" - on the other hand, in the past decade or so, many Canadians themselves have begun to refer to "the States" as "America". So there you go.

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It's my understanding that adding "'s" used to be the approved way of indicating plural abbreviations, but that the "rule" has changed within the last thirty years or so.