My guess as to the logical basis for plea bargaining is the notion that there's two types of criminals.
Type A criminals see nothing wrong with hurting others through criminal actions, and will not hesitate to do so if they believe it will get them material benefits and/or thrills (and think they can get away with it). They have no conscience or restraint. They are a danger to society and should go away as long as possible. Think of someone who kills a family member to collect life insurance money.
Type B criminals are basically ordinary people who are careless, make a bad decision, aren't aware that a certain action is illegal, or perhaps are consumed by emotional passion. They genuinely regret their actions and the consequences thereof. Think of a drunk driver who accidentally kills a pedestrian.
The theory behind the guilty/not-guilty plea is that Type A criminals will never confess, since they have no conscience or remorse. But Type B criminals will want to try to become a good person again, and the first step to this is admitting their past mistakes.
That's the theory. It doesn't always match reality. For example, the Type B criminal may be so troubled about what he's wrought that he blurts out a confession before getting a plea agreement in writing, in which case the state can still throw the book at him. Or an innocent person might confess as purely a risk mitigation measure: If you were some alternate universe's Aaron Swartz, you decided not to commit suicide, and you had a choice between pleading and getting guaranteed 6 months, or having a trial and a 50% chance of a not-guilty verdict but a 50% chance of 35 years in prison, you might be very tempted to choose the former.
Interesting mental exercise: I arbitrarily set the odds of winning a trial at 50-50. What do your odds of winning have to be before you choose to roll the dice?
To the degree that your contention is true, we can call a prosecutor "bad" who turns B's into A's through careerism or other motivation to intransigence. That some prosecutors will seek to decide for themselves who is an A and who is a B is an occupational hazard for them, and politically I would call it a form of mental illness.
I recall reading a critique of the US prison system which said it has a tendency to turn B's into A's. I'm not sure how true this is, and I can't quite recall the exact mechanism which was outlined -- I think it was something along the lines of giving B's months or years of 24/7 exposure to an environment where being an A is common and socially acceptable.
I say 50%; you say 80%. We could both be right. Here's why:
The odds you would use in the grandparent post's calculation would be different from the average conviction rate, because you'd have knowledge of the specific facts of your particular case, which would affect your estimate.
Despite all the noise HN's made about this case, I actually believe that most of the people who are punished by our justice system (1) are actually guilty of the crime(s) they're accused of (Aaron probably was), and (2) don't have mitigating factors that might cause the case to be dismissed by the judge when he/she becomes aware of all the facts, acquitted by a sympathetic jury, given a small fine or other slap-on-the-wrist sentence, or successfully appealed (Aaron's case clearly did have such mitigating factors).
Let's say hypothetically that, of every 100 defendants who go before the court, W=70 of defendants are rightfully convicted, X=10 are wrongfully convicted, Y=10 are rightfully found innocent, Z=10 are wrongfully found innocent. I'm using the term "innocent" here to lump together category (1) above, "factually innocent" (he/she didn't do it) and category (2), "morally/technically innocent" (he/she did it, but it shouldn't be a crime / sentence is far too harsh / prosecutor/police tactics were unconstitutional / jury didn't believe the evidence was good enough / good defense attorney managed to inject reasonable doubt / jury nullification [1], etc.)
Let's say you're being prosecuted. You know the above numbers (or they're your best estimates based on the available data and reasonable modeling assumptions). You know you're in the "innocent" category, that is, that you'll either be in X (wrongfully convicted) or Y (rightfully found innocent). Your chance of being wrongfully convicted is thus X/(X+Y) = 50%, as I stated.
But from the point of view of all cases, the overall conviction rate is (W+X)/(W+X+Y+Z) = 80%, as the parent stated.
The above numbers aren't based on any actual facts; they're made up to agree with both the parent's stated 80% number for overall conviction rate, and the grandparent's stated odds of a particular defendant's chance of winning of 50%.
These computations are an example of Bayesian reasoning [2].
That might make sense for type B criminals, but type A criminals may still be choose to get away with the lower sentence. And it doesn't address the issue of innocent (or mostly innocent) people who confess to a crime they didn't commit. I don't know how common that is, but that's a big deal.
Type A criminals see nothing wrong with hurting others through criminal actions, and will not hesitate to do so if they believe it will get them material benefits and/or thrills (and think they can get away with it). They have no conscience or restraint. They are a danger to society and should go away as long as possible. Think of someone who kills a family member to collect life insurance money.
Type B criminals are basically ordinary people who are careless, make a bad decision, aren't aware that a certain action is illegal, or perhaps are consumed by emotional passion. They genuinely regret their actions and the consequences thereof. Think of a drunk driver who accidentally kills a pedestrian.
The theory behind the guilty/not-guilty plea is that Type A criminals will never confess, since they have no conscience or remorse. But Type B criminals will want to try to become a good person again, and the first step to this is admitting their past mistakes.
That's the theory. It doesn't always match reality. For example, the Type B criminal may be so troubled about what he's wrought that he blurts out a confession before getting a plea agreement in writing, in which case the state can still throw the book at him. Or an innocent person might confess as purely a risk mitigation measure: If you were some alternate universe's Aaron Swartz, you decided not to commit suicide, and you had a choice between pleading and getting guaranteed 6 months, or having a trial and a 50% chance of a not-guilty verdict but a 50% chance of 35 years in prison, you might be very tempted to choose the former.
Interesting mental exercise: I arbitrarily set the odds of winning a trial at 50-50. What do your odds of winning have to be before you choose to roll the dice?