Well, no aside from "nautical miles" "ναυτικό μίλι" but if something doesn't exist it doesn't mean that we shouldn't learn it. And since Duolingo teaches American English we thought it only fair that we include it.
Perhaps because Greeks travel to and live in places other than Greece. I know that when I speak with Greeks here in the US, we use miles. Thanks to this coursd, I can now do so Greek. :)