About a year ago, I was on a train when I received a fairly nasty call from a scientist who I was profiling for a magazine, asking why I wouldn't allow him to check my copy before I sent it off to the editors. "It's my article," he told me on the phone. "No, it's not," I replied, much to his surprise. What he failed to understand was that an article about him, that included his thoughts, didn't actually belong to him. For him to read it and have approval would mean I'd be unable to criticise him or his work (whether I wanted to or not). See, that's journalism.
Yes, I know much science journalism merely explains a piece of research, a discovery or an idea, so I can understand researchers' desperation to make sure their thoughts are accurately expressed... but fact-checking stories can easily slide into censorship. Which is why I personally believe it is unethical to give copy approval, and why it is unfair for interviewees to ask for it. Incidentally, the above scientist later apologised, after he checked with his colleagues and they confirmed that he was in the wrong.