It's in our TOS and has been since 2003 (because most of our members do follow it literally)...
http://www.thrillnetwork.com/tos.php...ds#privmessage
We had to implement the solution, which included this hack, when we started getting complaints of spam and flaming via PM. Since we could not necessarily rely on having them forward the PM (they can be altered), and we actually record all evidence related to a suspension of an account in case it escalates, we needed to be able to go to the original source on our own. With two people who are comfortable with SQL (myself and my assistant techie), this tool is necessary.
Plus, when you think about it, a private message isn't necessarily private, especially when admins can just go into the necessary table, and we have never made any bones about it not being private, especially since members can (and have) taken content from PMs and pasted them in public, even against the sender's will.
Lastly, if you are a busy like we are, you do not have time to even think about snooping on PMs, and I'm too focused on other parts of the site to where my contact is email only, and when I am in the admin CP, it's mostly in the members (for bans and account fixes) as well as the templates to make a minor change to the look of our site or to prepare for an upgrade.