There are companies and individuals that pay people (work at home/freelancing) to register at forums/chat rooms/dating services/etc simply to spam private messages as well as the message boards with their propaganda. Some are legitimate (meaning not pornography, but not welcome), but annoying, much is geared to whichever subject matter they believe the forum population would be interested.
We've had this policy at a forum I mod, and a few I visit frequently (with much more stringent rules due to the heady subjects discussed) for a couple or so years now.