It's perfectly understandable for his writing style. He's (boy, I keep bringing this up a lot on debates lately) a very improvisational writer from what he has revealed in interviews, not really a plotter. That means that he has a lot of information and ideas in his head that he just wouldn't be able to do justice by putting them on paper. I know many writers who work the same way, and it's always troubling for them to hear alternate takes on their stories because all the 'what ifs' are mostly things they themselves have already argued about with themselves, crossed off, and never want to hear again so they don't influence their own creative process.
In addition, from his point of view he is also right about it being poor practice. Improvisational writers are focused on crafting a great world first and foremost, and style and technique are secondary. Therefore, taking someone else's setting achieves little in making a better writer from that point of view. I very much sympathize and agree to an extent.