"What if there were cameras in your house that recorded everything you did & said, but the government promised you the footage would only be reviewed in the case that they suspect you committed a crime. How would you feel about that?"
I've gotten a lot of mileage from this metaphor myself. The response is usually silence while they internally question their worldview or something. Never gotten a counter argument, nor anyone replying with "I'd be okay with that". Would be interested if HN could poke a hole in this so I can patch it. :)