"Witches and wizards, I give you Exhibit A: this glove I just Accio-ed from the accused ... "
Instead, the hodgepodge of guesswork and random whims that make up the Wizarding World's legal system are about as consistent as the rules to that game your older brother used to make up as he went along ("No, my farts are worth 10 points! I have the fart stick!"). For example, when an elf magically threw a cake at his aunt, Harry Potter was almost expelled from Hogwarts for it. That's because although the Ministry Of Magic can instantly tell when an underage wizard performs magic around regular humans, they can't pinpoint the caster. That's like someone getting arrested because a gunshot was heard in his or her neighborhood.
Let's table the fact that humans enslaved the single most powerful species in the Wizarding World.
But, OK, people get nailed based on flimsy evidence in our world, too. But then, there's Harry's godfather, Sirius Black, long considered one of wizardry's most notorious mass-murderers. In reality, Peter Pettigrew (the evil weirdo who pretended to be Ron's pet rat and who presumably saw Ron masturbate countless times) committed the crime Sirius was blamed for. But, it was far from an ingenious frame job, especially considering some of the tools available to the prosecutors.
First, there's the Priori Incantato spell, which is essentially like accessing your browser history -- it shows you the last spell a particular wand was used to cast. At one point, it gets used on Harry's wand and proves that the wand was used to cast an evil spell. But, even that wasn't considered sufficient evidence that he did it. However, the magic police don't even bother using this on Sirius -- the fact that he was present at the scene of the crime was all the evidence they needed to ship him off to prison indefinitely, because, apparently, civil liberties are right up there with human fashion on the list of things wizards don't understand.