They are not, by default vBulletin, the pluggin used to run this forum allows Super moderators and above (admin) to view PM's, now Sam and others in the past have said that setting was changed and only the tech guys (admin) who set the site up can read them but even so, all they've to do is ask for access and it's given, just a tick in the backend settings. They could even have reduced it and given site moderators access but that is unlikly.
Actually just checked, I see now all the moderators are Super Moderators, before it was a hierarchy and only 2 or 3 were super moderators, Natasha and I think Sam