PM's can't be read by anybody who doesn't have access to our database so that means moderators can't read PMs. The forum software itself doesn't have the functionality to just go poking around PM folders---and that includes administrators. Technically all data is all in our database but I have no interest in actually going through the effort of reading private messages. We're a small enough company that we can actually have honest policies.
Having said that if some day we get a call from law enforcement or a subpoena, yes, we might have to turn over PMs. But that's never happened in 21 years.