Mike, you bring up a great point and one that is argued quite a bit on many forums, however, NO private messages are truly private.
You won't see many admin readily admit that, yet it is true. I happen to be honest enough to let everyone know that and honest enough to not take advantage of that without due cause.
ALL vbulletin forums are stored in a database which can be accessed via the phpMyAdmin. Any admin who tells you they do NOT have access, is either lying or really does not know how.
Allow me to reiterate that it is like pulling teeth getting to the data without a proper hack but it can be done.
I see no sense in changing the name since it is private from the general constituency and would be unethical for any admin to share this information with any member or allow the power to do so to fall into anyone else's hands.
Are all admins ethical? Well, if I found out he/she wasn't then I would be gone from that forum yesterday
So yes.. technically they are not "Private" exactly according to how Mr. Webster defines it but the tool was designed as a way to "post" a message without everyone and their brother seeing it like they would a normal post.
In the phpBB forums, the control panel has the "View Private Messages" as a standard feature in the Admin control panel if I am not mistaken.