The term "Private Message" is a general term widely used in discussion board and blogger circles as a means of distinguishing between public and private comment. It is also the term built into the UBB software that drives the MS forums. This software is purchased as a package from a vendor and can only be modified within the parameters set by the developers for the end user of said software.
The term Private Message is not within those parameters and as such is pretty much set as is.
“Life’s journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy ____…! What a ride!’” ~Hunter S. Thompson