> those are generally people much more relevant to the US than Khan.
That's debatable. I've seen lots of statues of, for example, Poles who came to America to help fight in the Revolution (Casimir Pulasky and whatnot). These people ahve a tendency to be almost *entirely* irrelevant to the bulk of American history... certainly nowhere near as important as Ghengis. Had some minor European army Major not come to the US and gotten shot dead in his first battle, the US would almost certainly still exist, perhaps with no recognizable changes. Had the Khans not existed, the US probably wouldn't either... except as part of the greater World Caliphate.
Nevertheless, these minor Euro figures are relevant to the people who actually paid for the statue - typically the Poles in Chicago, the Italians in New York, whoever.