Authors using other author's characters

A couple of days ago, Patti over Pattinase posed a question about authors writing fiction using another author’s characters. It’s not really bothered me one way or another and it’s not something I’m particularly interested in doing myself, but we’ve been house bound for the past few days due to the weather, so we’ve been watching some DVDs – Morse and Frost. So Patti's question got me thinking about these series. Whilst some of the episodes are based on the novels of Colin Dexter and R.D. Wingfield, others have been written entirely by other writers using the characters in their own story. The entire run of Frost consists of 38 episodes, yet Wingfield only wrote 6 novels. Similarly, there were 33 episodes of Morse but only 13 novels. It’s pretty difficult, to be honest, to know which episodes were based on the novels and which were written by completely different authors. Clearly in TV land, having lots of writers scripting stories using someone else’s creations is not a problem. There seems to be something different about using them in novels, but I’m not really sure what or why? Something about TV not foregrounding the author maybe?