If I own a cookie store, and you come to my store for a job interview, and I see you stealing cookies, I will not hire you. To me, you are a crook, and I will act on that belief, and I can do that. But, if I turn you into the cops, and you enter the criminal justice system, you have a presumption of innocence, which applies to what happens to you in that system. At no point does the presumption of innocence that is part of our criminal justice system seep out into my cookie business. I caught you with your hand in the cookie jar, and you still can’t have that job.
This rule of justice comes down from the Romans, Ei incumbit probatio qui dicit, non qui negat, and it is widespread, though not universal in legal systems. But it does not apply to my cookie shop.
The presumption of innocence is not a logical requirement or even guideline for how humans think. It is, rather, the baseline from which the process of proving guilt begins in many of our legal systems. “Presumed innocent until proven guilty” (a phrase coined by William Garrow) refers to a stage in the legal process of acquiring or convicting someone. It is not the logical starting point for deciding, as a human being, if another human being did something wrong.
I do think something like presuming innocence is a good thing to do, and I tend to do it. But when as citizens we do that of each other, or of people in the news, or others, we are not engaging in the legal principle of Ei incumbit probatio qui dicit, non qui negat. We’re just trying not to be dicks all the time.