Cultures in America that don’t know Christmas is part of the larger American culture are extremely rare. I’d argue they’re non-existent. It’s like going to Germany and telling them to knock off the Christmas markets, there are Turks around.
They may know but it still may not be gainful to virtually parrot “Merry Christmas” to them. I wouldn’t order my clerks to do that to a clientele of Lubavitcher Jews, for instance; they well know what Christmas is but the wish is utterly wasted.