lpq's Journal: Who owns rights to public domain?

Saw this article where I read that "a representative for Edgar Rice Burroughs' estate said Disney was acquiring the rights for a live action film" for "John Carter of Mars", released to novel in 1916. At least the first books in the series are in the public domain (copyright expired) and is available for free on the Gutenberg Project. What "rights" does one need to "acquire" to produce such a movie? Is this some sort of public "sham" to convince the public that Disney owns the rights to something in the public domain?