I think it’s a mistake to equate the United States with “America”. I prefer calling Americans “Yankees” or “Usonians”.
I feel very confident that Canadians do not want to be referred to as Americans.
"American" or "America" in this context is short for "United States of America" and has been used that way for generations, here and around the world. For the geographical term, use "North America," etc.; for the cultural, perhaps use "Anglo-America" or "Latin America."
Guess who live in the United Mexican States? You guessed it, Mexicans!
Guess who live in the United States of America? You guessed it - Americans (and a lot of others).
The current US Government has added a whole new level of meaning to the slang, as a septic tank is typically full of shit.