How could such an NDA be enforced? You generally wouldn't have any way of proving who leaked the question (unless you ask every candidate a unique question), and even if you did spend the time and expense to litigate against the leaker, it would be hard to convince a jury that disclosing the question would have incurred a non-trivial loss for the company.

If an interviewer asked me to sign an NDA, I'd think the company was run by a bunch of crazy control-freaks and walk out.