That seems at least slightly unreasonable. I think it's appropriate to ask the candidate to write pseudo-code (or real code, depending on the problem), especially if you are just examining how the candidate thinks through problems. I'm perfectly comfortable doing this during an interview, but, then again, I often sketch things out on paper. But, different strokes for different folks.

In the past, I have been asked to sit at a machine and write code, which was far better than doing it on a whiteboard.