Shame they had to shoe in American accents.

You assume wrong. Other British series had been successful in the US without an American star. There was a feeling in the industry at that time that having an identifiable American star would improve the marketability. Whether that was true or not who knows, but the bottom line is that TV is a business and obviously ATV and Brian Clemens wanted to do anything to improve the chances of success. I watched all of these episodes in the original run in America on ABC late night. I wouldn't have felt any differently had there not been an American in the cast. At the same time I think that having an American does work to its advantage at times, especially with the fish-out-of-water aspect of some of the plays. What's interesting is how accepted it is today for American produced shows to have English or Australian actors (either playing their own nationality or pretending to be American), yet we still have very few series imported from the UK.