In the United States, TV writing is work-for-hire. The studio owns the property and everything created for it. That's standard in every contract. It's different in the UK, where freelancers do generally own the characters and concepts they create (which is why the co-creator of K-9 for Doctor Who was able to do an Australian K-9 series without the involvement of the BBC). That, I assume, is why the fine print at the end of American movies and such says something like "So-and-So Studios is considered the author of this work under UK law." There's no reason it would be WGA policy; that makes no sense, because the WGA is a union that advocates for the writers, not the studios. If it were up to the WGA, probably the writers would own their work.