While the American Memorial Day was originally honoring only soldiers who died fighting for the United States in the Civil War (important, but we didn’t have a horse in that race), it was expanded to cover additional wars. In more recent wars (World War II, against the Nazis), the Cold War (against the Communists), and the current “War on Terror” (against the Muslims), the Americans were fighting our enemies, and an American defeat would have been very bad for the Jews. While strictly Orthodox Jews are largely excluded (de facto) from serving in the American military (at least in uniform, plenty of frum Jews serve as civilian employees), we have reason to honor those who have died fighting for the United States, since in many ways we are the chief beneficiaries since more than most Americans, we would have fare poorly in a world dominated by the the likes of Hitler, Stalin or Bin Laden.
If you are a zionist, one certainly should honor fallen Israeli soldiers. If you are not a zionist, it is best to keep quiet since it goes without saying that if the zionism is a mistake, those soldiers died fighting for a big lie, but it is rude to remind the families of those who died of that.