I agree .... Good question. In all my years of hunting for merchant counterstamps, I don't recall seeing USA coins, other than
Trade dollars and an occasional
Morgan dollar, that had oriental chopmarks.
My understanding of "chopmarked" coins has long been that this term refers specifically to world (USA included) coins that circulated in the orient and were stamped by merchants there. Their chopmark labeled the coin good silver, in effect.
I have a coin dealer friend who's married to a Chinese lady. A few months ago, I took one of my few chopmarked
Trade dollars to a show so that she might be able to enlighten me about the Chinese characters. She studied the marks intently, telling me that they represented different family names. She noted one, apparent Japanese chopmark. She was only able to generalize about the marks, being unable to answer any who, what, when or why questions. Surprisingly, to me, she, wife of a coin dealer, was unaware of this practice of chopmarking coins.