The mode of transport the RF use, or there housing is unimportant in the grand scheme of things. Whether people like it or not they are the Royal family and they do things differently than us mortals, just like the US President, or the Swedish royal family. Helicopters have come a long way and modern ones are far less of a climate issue. It makes sense to fly in a helicopter a short distance than all the fuss of arranging convoys of diesel cars or even a whole train. Time is valuable when you are rich and important, and again, like it or not they are.
William was in New York and visited a Fire Station and the crowds were large and enthusiastic. I doubt these visits have a lot of prior warning because of the risk of terrorists, so it was nice to see him interacting with the crowd. Great that he is so popular in America, and can focus some light on his Earthshot project.