That is an interesting question, VM. I also found the follow-up question in the case of "Pastor" Russell interesting too. I hope those with real information would see the question and offer answers as I'm really curious.
In my years in the religion I remember that he was called "Judge" Rutherford because he did occasional work as a substitute judge while he worked as a lawyer - a public prosecutor specifically. And even when he became a "Bible Student" (as they were then known) he became the Watch Tower's legal counsel. As far as I know he never served on the bench in any court since becoming a Bible Student.
While the Watchtower publications give a rosy picture on why he was called "Pastor", from what I can find his name came about because congregations at the time relied on membership voting their church to be associated the Watch Tower organization on a yearly basis. They would vote Charles Russell as their "Pastor". Soon other congregations would adopt this title as a tradition and eventually he earned this nickname which eventually stuck through the rest of his life. He didn't mind it at all. But the fact is he never recieved any theological education nor certification from any institution.
I hope this helps but I yield to those who know more than I do.