As for the principle that elected representatives of the American people should be able to travel where they like — the principle enunciated a few weeks ago by Ron Dermer, Israel’s ambassador in Washington — Mr. Trump has made his rejoinder clear: Principles are for suckers. How many dictators are rejoicing today that a U.S. president has given them full permission to bar members of Congress who in the future might want to visit their countries to monitor elections or speak up for human rights?