"No jury would convict for one joint, so it sounds like a plea bargain..."
You are probably right, but that doesn't mean it was a good or fair bargain, or even that it advanced some good public policy. The kid (and his parents, if they were involved) may have been told the maximum penalty possible if they went to trial before a jury, but did not realize how unlikely a conviction would be, so they took a definite fate rather than an indeterminate one. If, as I have imagined the scenario, they voluntarily took a deal, do they then have no cause for complaint? Maybe so. But what about the effect on the rest of us? We have to pay for this kid to be incarcerated for a year or more and then pay for the follow up probabation and parole. If his life is ruined by having been in prison, then that will cost us still more (in public money spent and his productivity lost). It's easy to say that he shouldn't have done the crime (and he shouldn't), but shouldn't we balance the costs of incarceration and its aftermath against the damage caused by the crime? If the two get too far out of proportion with each other (which may be the case here), then you have a miscarriage of justice.
At the Taconic Parking lot, Jose asks Mitchell if he has any smoke. Mitchell thought they were going to merely get high. He pulls out a bag with enough marijuana to only smoke one joint, one cigarette.
Jose asked to buy it and Mitchell told him it's not worth a purchase.
He insisted and would pay $20 for it. However, Mitchell wasn't comfortable with this transaction seeing as he was not a drug dealer, but a pot smoker.
Jose tried to get Mitchell to sell him a bag of marijuana, again. This would solidify that Mitchell was indeed a drug dealer or at least in the eyes of the law it would have.
Mitchell wouldn't do it and told him why. He assumed that was the end of it. Until a few months later when he was arrested at his home.
There was a 3 day trial, the judge would not let the jury consider entrapment. Nor, were they aware if they found this 17 year old guilty, that he would be sent directly to prison.