Why "Toy Story 3" is Nominated for "Adapted Screenplay Oscar" and Not "Original Screenplay"

I've seen this question asked around the Internet quite a bit.  Screenplays for sequels are considered adaptations because they are based on the original story or, in this case, film.  Toy Story 3 uses characters featured in Toy Story and Toy Story 2 and it is, of course, a sequel.