They consider it a form of Spam and they do have a point. Every 'regular' chat site would become a spam factory if it would be allowed. I remember when I used to get tons of signups through Yahoo clubs back in '99
That's now considered spam as well. The point is that these services are simply not meant to be used as porn promotion.. If surfers want porn they'll find it themselves and it is our challenge to create sites that attract surfers that look for porn.
Good luck