Why does CBS advertise on CBS?

CBS airs a lot of commercials for its own shows. Why?

At first this doesn't seem like a puzzle. Why not? They don't have to pay for it!

But every minute of air time they dedicate to their own commercials is a minute they can't sell to customers. The opportunity cost of advertising their own shows is probably substantial.

In fact, if the TV advertising industry were perfectly competitive then CBS should pay the same price to advertise on its own network (with a cost of MC) as to advertise on NBC (for price P) since P = MC.

Does the same argument hold up if broadcast TV was monopolistically competitive? Can you explain how CBS gains from filling time with its own advertisements?

P.S. Do cable channels advertise their own shows as heavily as broadcast channels do? I don't pay enough attention to notice but why might we expect cable to be more competitive? Why might we expect the opposite?