Is it really because of privacy though, or is it because of reciprocity?  Do people not get prosecuted for trivial law-breaking because the relevant authorities don't know about it, or is it because they're afraid that if they set the precedent that these laws will be enforced, they wouldn't want to live in the world that results?<p>It's always been fairly easy to dig up dirt - even felonies - in peoples' pasts.  All you need to do is find one of their childhood friends, act all buddy-buddy, and buy him a few beers.  Then as in now, these usually only get prosecuted if somebody has a vendetta against you.<p>Think of the times that people commit crimes right in front of a police officer and walk away.  Haven't you ever driven a few mph over the speed limit when a cop pulls up behind you, think "Oh shit, there goes my insurance premium" as you pull over - and then watch him speed past you, because he's got something more important to do than write you a ticket?  When I was in college, the campus police would regularly turn a blind eye to underage drinking right in front of them, as long as no property was being destroyed and nobody had alcohol poisoning.  When teachers show a movie to their class, does anybody actually believe that they've acquired public-performance rights to the movie?<p>In polygraph tests, questions on these sorts of crimes are asked as a control, on the assumption that <i>everybody</i> lies about them.  The theory is that a normal person will be more nervous about the little crimes that they're sure they must've committed at some point in the past (even if they can't remember exactly when), but will <i>know</i> they didn't commit the particular crime they're being accused of.  A real criminal will be more nervous about the big crime they just committed, and all the petty misdemeanors pale in comparison to that.  Makes me wonder if polygraphs give false results for those saints who don't actually do anything wrong.<p>It's not lack of information that stops most people from being prosecuted for giving our mothers a couple of pain killers, it's that most of the people doing the prosecution figure that they would do the same thing.