It's sort of how USians say "zip codes" for foreign addresses when they mean "postcodes" (the Zone Improvement Plan, from which ZIP is taken, is a US thing).
A lot of languages have a word like USian, e.g. "états-unien" (French), "estadounidense" (Spanish). I have used "USAmerican" myself when "American" would be ambiguous.
That's us. It's our word. You can pry it from our cold dead lips.
I am not sorry if my attempt to use precise language when precision is needed is "disingenuous" to you. What a weird thing to get rude about.