They need to get usernames from somewhere, since there are probably a few thousand active users out of the 77,000 registered users, and it doesn't make sense to PM thousands of dead accounts. The easiest place to get usernames is the list of who is online at the bottom of the main page. By setting your online status to always show offline, you don't appear on that list. It prevents most of the PM spam, but not all of it. If you post publicly, they can find you that way too.