I got a very strange look from the Princess Royal last year when I bowed from the waist as she passed at Trooping the Colour. Nowadays it isn't the custom to bow from the waist although I always have when I've met members of the Royal Family. She looked a little taken back and then smiled. I think she must have appreciated it that people were still loyal enough to show courtesy. Or maybe she thought she was in Japan. I just don't know.
Sorry Darlin', at no point were we ever expected to bow or curtsey. People did so if they wished. I was in England for the Jubilee, in London, both nights for the Prom and the Party and then the Parade. When her car passed down the Mall both ways I did not notice a lot of people bowing or curtseying.
Just because you are British does not give you the right to imply a demand of Americans bowing or curtseying.
It never has been customary to Bow/Curtsey while HM was in a car or carriage, the custom only applies when the Sovereign passes in person such as in a procession down the aslie or when actually meeting A member of the Royal Family in person. I am not demanding that Americans bow/curtsey but to me it just appears churlish.
We don't bow or curtsey period. It is not required, expected, demanded, requested, asked for, wished for, etc.
While the irony of how we separated from England at the time is clear in the sense of the present American interest specifically in the British Royal Family over the years is obvious. Her Majesty, being the intelligent and pragmatic woman she is, knows better than to expect anyone from any other country outside her realm to bow or curtsey.
It's probably just as well that Americans don't bow or curtsey. Can you imagine bowing or curtseying to a different Hollywood flavour of the month. I recall listening to a TV report about Prince William which the reporter concluded with, "We Americans may not have a Prince William, but we have our very own princes like Brad Pitt of Hollywood." I laughed til I fell off my chair. :p