Emily Post...Sheesh.<quoted text>
So if a person earns a doctorate degree in law, you DON'T call him "doctor", but if a person earns a doctorate in medicine or political science, THEN you call him "doctor". I once had a music teacher who INSISTED you call him "doctor", and I had a science teacher who had a doctorate degree in physics who insisted you call him "Mr." and NOT "doctor". Who makes up these rules ?
These rules are very old. In the United States, one who happens to have a juris doctor is usually referred to as esquire. Such Kate Kendall, esquire of Lambda Legal.
It's all a matter of etiquette and protocol. A person who earns a doctorate of philosophy or a doctorate of medicine is referred to as Doctor. A person who receives a masters in arts, sciences, and such use to be referred to as Master or Mister, but that's fallen out of favor. A person who earns a rank in the military is referred to that rank even after retirement.
Basically, society is the ones who decided all of this.