ABC obtains rights to new Mike Judge animated series

Most don't. There are some producers that are able to use non-union writers but most of the people who write for animation are represented by the Cartoonist Guild (actually an IATSE local) because at the time the guild was set up animation was mostly done without scripts through the storyboard technique. That's no longer the case today and although there hasn't been a scientific poll it's believed that most animation writers would like to be part of the WGA, in part but not only because the WGA writers get residuals while residuals earned by IATSE members get plowed into the union pension and health plan for all members.

Thanks for the informative reply, Brent.