The point is we don't really know who is innocent or not, because the incentives are so fucked. If you're poor and need to get on with your life, you take the guilty plee almost every time. Trial takes fucking forever, and it's very expensive.
What this means is the you can be charged with almost anything, and the odds are very high you will plea guilty, regardless of your innocence. There's basically no incentive for the police or prosecutors to show any restraint, they have a "get out of jail free" card in the form of plea bargains.
Let me make up a number. 7%. I think that number of plea bargains would be a huge problem if in 7% of cases the person is in fact innocent. Would you disagree?
And even generally assuming guilt, a number that high gets worrying. Maybe we're only prosecuting the strongest of strong cases or something, but some of the other factors that could be reducing the rate of trials are really bad for justice.
What this means is the you can be charged with almost anything, and the odds are very high you will plea guilty, regardless of your innocence. There's basically no incentive for the police or prosecutors to show any restraint, they have a "get out of jail free" card in the form of plea bargains.