Many american shows are loved in Britain and Europe. This is an example of a british show that was beloved by many in the United States. And it was not without American references. The one episode with the Americans (Waldorf Salad) of course. Also, Basil seems to reference Henry Kissinger a lot, who was U.S. Secretary of State during the first six episodes of Fawlty Towers. He also wonders if Mrs. Johnson is the late president's wife (Lyndon B. Johnson).
This no doubt helped. When Cleese starred in a Fish called Wanda, a very popular show in America, he was more of an American Icon than Jamie Lee Curtis and Kevin Kline were.