Networks decide when and if a new episode of a television show is made available On Demand on Comcast. They also receive the TV ad revenue. Some episodes become available for viewing the day after originally airing on television and others don't become available for a week or more.
For example, FX allows for new episodes of American Horror Story: Asylum to become available for viewing On Demand eight days after they originally air. Their licensing restrictions also only allow three episodes to be On Demand at one time. AMC makes episodes of The Walking Dead and The Talking Dead available On Demand just one day after originally airing on television. Four of the most recent episodes of each show are available for viewing right now.
Of interest, fans of The Mentalist are in an uproar after discovering that CBS is not allowing the show to appear on Comcast's On Demand platform. The show's publicist has announced that three of the most recent episodes are now available for viewing here.