Trying to find logical excuses for any retail fuel price, let alone the spread between diesel and gas is sort of like developing a perpetual motion machine, it can't be done. The answer to the question is "because they want it to be a dollar difference". The difference between diesel and gas in Canada is much closer, with some areas selling diesel for less than gas, why? China doesn't like their diesel?, I don't think so as the US gets the majority of it from Canada, the answer is they price it where they want to.
Higher diesel prices are more likely to be accepted by end users because most diesel is used in industries like trucking for instance and they recoup much of the price through fuel surcharges. If those industries were not able to pass the cost along you would see a much different price structure as well as a lot of noise from various lobbies representing trucking, agriculture and so on. It would be nice to be able to put the blame on someone far away around the world, but the reality is not so. If blame is to be placed anywhere, the first place to look is our own neighbors in the US oil companies.