Some poker rooms / networks do not allow rakeback - like Party Poker, Ipoker, etc. Thus the simply act of "advertising" rakeback for this rooms is forbidden. For some rooms there are still private persons offering private deals called "under the table" deals most of the time.
This deals might give you the chance to get something that is not allowed, but also leaves you with a higher risk of course.
Hope this explains it... donīt know how to say it otherwise.
edit: as always Bobo is first...