Surely it is the person/company doing the hiring who has the say in what is and isn't required for the position? Assuming the interviewer isn't doing something that's blatantly illegal ("Do you plan on having children in the next x years?", "I think you're too old for this position" etc.) I don,t think an interviewee is really in a position to argue this as surely they'd almost always have very limited knowledge of what the position actually entails compared to the person doing the hiring.

Exactly, no interviewer would want to have such a discussion with the interviewee. So their best course of action would be to smile and say "I don't have any concerns, thanks for asking."