You misunderstand. The USA does not have a list of Japanese Citizens, meaning that if a Japanese citizen decides to become and American one they have no concret

You misunderstand. The USA does not have a list of Japanese Citizens, meaning that if a Japanese citizen decides to become and American one they have no concrete proof that he discarded his proof of Japanese citizenship (in this case a Passport) unless they reach out to Japan. Unless you are an international criminal the US will not waste time or resources requesting that information. This is true for most countries.