If you're in an exit interview and you're asked for your opinion on how your boss treated you while you worked there (regardless of whether it's your own boss asking the questions or not), you can't just say nothing... uncomfortable silence is uncomfortable.
stand in front of a mirror and practice this:
"I'm sorry, but I have nothing to say about that issue."
sieze your own power. do not give it away! its their right to ask anything they want; and its your right to not answer anything you don't want to.
restated: because someone asks a question is no reason to obligate you to respond to it.
its hard. but its a skill we all should try to master (better).
... right. Because helping HR improve their company, after they gave you a job and paid you to work there (at a salary you agreed to, no less!), would be a bad thing. You might think you were underpaid - but you weren't forced to work there.
Only an american would claim that they've got the right to say whatever they want (or nothing at all) AND ignore the social responsibilities of participating in a free and open society.