^I figure it's just the old myth that only "original" creation is worthwhile and working on an idea that somebody else worked on first is inferior. Which is ridiculous, since every work of creativity is inspired by earlier works, or a response to them in some way. We all build on the work of others, even in our original work.
But you're right, it is an odd double standard to dismiss novelists who work in a franchise universe but not apply the same stigma to writing staffers or freelancers writing for the same franchise on TV. Although another of the pervasive elitist stereotypes out there is that TV writing is inferior to literary writing -- which is ridiculous, since some of the finest writers alive today, like Aaron Sorkin, work in television.