Well, that ignores certain practicalities.

Sitting in jail if you can't make bond (which would be the case for lots of the casual users) and losing your job isn't a viable option.

Having to go through a lengthy (and expensive) trial process just to get to the jury isn't a enjoyable option...and there's always the chance that you won't get that 8%.

In many jurisdictions it's classified as a misdemeanor with a shorter sentence, which means you may only have 6 jurors, so the odds just dropped.

And of course, a huge number of these cases settle before trial with a reduced sentence (or just probation) which still results in a criminal conviction.

Let's set up a realistic scenario, at least for many jurisdictions across the US: If you're busted and charged with a felony and facing 5-8 years, it could cost you $30,000 to $50,000 or more in attorneys' fees and court costs to get to a jury, which might take a year or more, and you're still rolling the dice that you get the jury that wants to nullify the law (which also means those jurors are willing to disobey the judge's charge to the jury). If the prosecutor offers you 120 days in jail on weekends and weeknights (so you can keep your job) and a small fine plus some community service and five years' probation, what are you going to do?