Have never thought about it before, but while I am right handed I always hold knife in my left hand and eat with right hand. Cutting prepared food with non-dominant hand never felt like a huge task since what you usually cut is easy to cut, it’s not like you are trying to cut a thin slice from huge piece of beef.
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Just kidding and not American. If saying ″thanks″ for things like those would yield similar reaction, I would be confused as well. Seems so intuitive to say it.