>90% percent of defendants plead guilty before trial because 90%+ of defendants are actually guilty of the crimes they've been charged with. In most cases, the defendants accept their guilt and are simply trying to get the best deal (i.e., least prison/jail time) as they can.
Except that a disproportionate percentage of poor defendants (who can't afford expensive lawyers) plead guilty before trial because their overworked and underpaid public defenders advise them to.
Logic is that pleading guilty before trial gets you a shorter sentence than going to trial. The alternative is going to trial and being incorrectly found guilty and getting a larger sentence for a crime you didn't commit (your public defender has limited time/resources to make a great case for you).
The American Justice system is particularly unjust to the poor.
Edited for clarity
No, a disproportionate percentage of poor defendants plead guilty because they're actually guilty and their underpaid public defenders have advised them its not worth the effort to fight the charges--for the PD or the defendant.
Especially since, as you helpfully pointed out, the DA is less willing to give shorter sentences to guilty defendants who demand a trial. In this context, "shorter" meaning shorter than the likely sentence imposed by the judge after trial, usually even shorter than the statutory range imposed if the defendant is found guilty at trial. This is why defendants plead before trial--in many jurisdictions its the only way you can get a sentence less than the range unless you have a fantastically expensive lawyer (like the lawyer for Brock Turner, the Stanford Student Rapist).
The alternative is going to trial and being incorrectly found guilty and getting a larger sentence for a crime you didn't commit (your public defender has limited time/resources to make a great case for you).
You appear to have a Law & Order understanding of how criminal defense works. In the real world, even private attorneys spend most of a case attacking the DA's case, not building their own case. The Defendant doesn't need to prove anything.
Every part of the American justice system pushes people towards confessions:
a. People are sent to jail pre-trial. This automatically disrupts their lives, as even a few days in jail will often cost them their jobs. [1]
b. People can be kept in jail for months pre-trial, costing them their houses and possessions as well.
c. The government can freeze or seize a defendant's assets (effectively without cause), rendering them incapable of defending themselves. They can (and do) hold these assets hostage to coerce a confession. Thankfully, the worst of these practices have been struck down recently [2]
d. Being unable to pay for your own defense lawyer gives you at best an overburdened public defender (poor defense), and at worst no legal representation whatsoever (no defense) [3]
e. Bail eliminates the only remaining way to avoid jail for the poor or those with frozen/seized assets [4]
f. Overly harsh penalties make it too easy to completely destroy someone's life forever. Even 10 years is ridiculously high, especially considering there's no path to reintegration into society in America. You're pushed outside and left, destitute, to your own devices. It's far preferable to start with a scarlet letter now than in x years, older, less employable, with zero assets.
g. Plea deals, while well intentioned, have become de-facto extortion due to the other problems in the American system. "Take the deal or else we'll keep you in jail, and maybe even tack on a bunch of extra charges to keep you here for decades". Even without this problem, plea deals are a corruption of justice. Either someone is guilty and deserves the punishment, or they don't.
A conviction rate above 80% is automatically suspicious, simply because no police department is that efficient. America's conviction rate rose from 75% in 1972, to 93% in 2012, and is still rising. Couple that with an incarceration rate rivaled only by authoritarian regimes, and I'm VERY suspicious of statements like "90% of defendants are guilty".
[1]
https://www.americasquarterly.org/aborn-prisons
[2]
http://www.abajournal.com/magazine/article/court_says_crimin...
[3]
https://www.cbsnews.com/news/inside-new-orleans-public-defen...
[4]
https://eu.tallahassee.com/story/news/2018/08/23/federal-jud...
https://eu.usatoday.com/story/opinion/policing/spotlight/201...
The prosecutorial advantage is even bigger in grand juries (ie. the juries that decide whether the suspect will be charged or "indicted" with a crime), where only the prosecution's side is heard and the standard used to indict is much lower than the "beyond a reasonable doubt" standard used in a real trial. The running joke being that a good prosecutor could indict a ham sandwich.
It's after indictment but before trial that the prosecution pressures the defendant to take a plea deal (ie. plead guilty to reduced charges) to avoid risking going to jail for a much longer time and bearing the insane expenses of a trial.
To my memory, in the US only something like 6% of indictments actually go to trial, where (as you point out) the prosecutors have a huge advantage. 94% of the time inicted defendants plead guilty and never actually get a trial. In some cases, like with Aaron Swartz, they commit suicide.
So for the overwhelming majority of cases, it's not what happens at trial that matters (as they never get a trial), but rather what happens in the even more skewed for prosecutorial advantage grand jury that matters -- a grand jury where defendants and defense attorneys are not even present and where their side is not heard.