There is no easy way to communicate with an HPV rider if you are outside it and it is moving. The way they let you know they are passing is by making a piercing sound that is not directional (left/right). I'm all for HPVs getting a clear run (before everyone else if need be), but can understand the average joe / jean (mentioned as "market research") feeling intimidated by them. I can also see scenarios where they would have to interact with the other riders, and yes, they do go a lot quicker, have much better COG, are less maneuverable, and truth be told, there are inconsiderate people everywhere, not just on uprights.
The goal of the ride was to raise money and awareness. There are far more upright riders than HPV riders. If a sample from a very large target market (uprights) are discouraged by a very much smaller market (HPVs), you reduce or remove the smaller market. It's purely marketing / economic sense.
I reached this conclusion by assuming the email you received was correct or the truth, and looked for reasons supporting that truth. This is not a decision I necessarily share or condone, and the promise of looking at HPV inclusion in the future is again something I would accept as the truth. Getting upset about it is understandable, but wild theories (embarassment) will probably not help your cause.
I would not want scooters, motorbikes or solar powered bicycle contraptions involved for similar reasons.