Courts (read: prosecutors) routinely use legal blackmail to coerce defendants into agreeing to plea deals. The threat is "we will prosecute you, and add extra charges, and push for maximums, that is unless you agree to these terms".
And those terms, as others have rightly pointed out, can include punishments the court normally isn't permitted to ask for on sentencing.
Also, with our judicial punishment based system, and that those with more money can afford better lawyers. And those with less money get public defenders, who are well known for not doing their job, or the absolute minimum to keep from being investigated by the Bar.
The only way out of here is to ever avoid interacting with police or courts. Once you're in that system, any sympathy is thrown out the window, and you become a money-pinata for the state and private 3rd party companies predating on your socio-economic class.