I was reading a blog post that talks about exactly how much the author is able to put in the public domain. My understanding is that Willingham has a fairly individualized contract with DC that he is grandfathered in on and is rather abnormal nowadays and gives him more control. DC has been trying to, as stated above, “reinterpret” that contract to give them more control.
Essentially, DC may own the rights to the individual products they published, but the world and characters Willingham created can be used outside of those in new or reimagined context.