I think Rumsfeld gets credit for it because he was using it in the most degenerate, disingeuous form possible. Rather than guarding against legitimate concerns and pursuing actual handling of potential issues, he was just trying to rationalize continuing policies that were demonstrably counterproductive. It's one thing to say there might be factors we don't know. It's another to day that simply because there might be such things, we should dedicate significant resources and lives to blindly flailing away under the assumption that it will help. A presumption of unknown unknowns puts you in a position of not acting, normally. There is no way to know that you aren't exacerbating and making a problem worse if you know that little.