Although I have no direct experience or examples to point to, my hunch is that registering the names of existing TV shows could open the door for legal hassles, ie TM infringements. (<- Relative to resale)
Motion pictures have successfully sidestepped the, potentially, high purchase prices of domains bearing their film's name by appending words to the name. ie Example"TheMovie", Example"Movie", Example"TheFilm", Example"Site", Example"Website". If motion pictures have resorted to using workaround names, TV prod cos and TV advertising depts w/ the studios, which are, generally, stingier w/ their budgets, would, more than likely, do something similar if they felt they were being jammed up. (ie in the event that their .TV name had been registered, was inactive and non-income producing.) IMHO