A reason the providers advertise on there own service is because people who don't have that service will see it when they go over to a place that has that provider.
For example, you have D* your family member has E*. Your family member comes over to watch a baseball game. You both sit down and turn on D*. During a commercial D* advertises MLB Extra Inning. Your family member might become interested in switching to D* because he is a baseball fan, and he would like to follow his favorite team. He can't do that with E*. The EI commercial will state how someone can follow their team all season long. It will also give the introduction promo price for a new subscriber. And you tell them if he switches over, you can do the $100 refer a friend if he does switch.
D* gets a new subscriber, both of you get the $100 refer a friend credit, just because he went over to your place to watch a ball game. That is why the providers advertise on their own service.
The Aflac commercials with the duck are entertaining.