What if the interviewer refuses to do it. You walk away? And remember, you're the one looking for a job and applied there (unless you're jeff dean, of course :)
When enough candidates walk away, it prevents the company from doing productive stuff. Not only did they not hire someone who could do something, they also wasted so much time interviewing instead of doing productive stuff.
They'll feel the heat and hire a reasonable candidate.
They'll feel the heat and hire a reasonable candidate.