Well, that’s because it makes no sense. “Illegal immigrants” means they BROKE the federal law. So it makes no sense to say “it would be up to the states sas long as they did not break the federal law”.
Which is the real problem here — Cain’s answer just doesn’t make sense. Cain really needs to stop re-interpreting the questions asked of him, and just go ahead and answer the actual questions, rather than arguing about it.