TBF,, your example is more a political interpretation than an urban legend. A lot of high school freshman classes use the Wizard of Oz as an introduction to allegories. It can be viewed as a metaphor for the social,political, and economic events of America in the 1890s. I believe that's what you're referencing. An urban legend about the movie, for example, would be that one of the munchkins hung himself from a tree and can be seen briefly in the film.