I think that it's more like a surgeon is called "Doctor", and when a Ph.D. in Physics walks into a hospital, they are called "Doctor" too. The Ph.D. would inform someone of what his training is in if it comes up, not to mislead anyone to think that he is a medical doctor, but the symbol of accomplishment carries over from the university building to the clinic. I think you are absolutely right. It about respecting the level of training that the other person has gone through.