At the core of the legal system is a fiction. We all know it to be a fiction, but without it the system would be incapable of functioning, and so in the interest of maintaining a system to resolve disputes that doesn't involve swords or pistols at twenty paces, we accept the fiction as necessary and turn a blind eye to its existence. The fiction is that the players in the system representing the power of the Sovereign are inherently trustworthy.
From the police officer on the street, to the prosecutor in the well, to the judge on the bench, we accept their word because to do otherwise would be to have no starting point for a credible system of justice. We presume good faith. We presume integrity. We presume that they execute the functions of their office with integrity.
Why is this a fiction? Because they are all human beings, clothed in their respective official positions. They carry the same baggage that all other human beings carry, prejudice, misperception, ego, antagonism. Like all human beings, they are flawed. But to admit that we've put the system in the hands of flawed human beings is to concede that it is impossible to craft a system that has inherently integrity. Instead, we would admit to a system that is no better than the players upon which is relies, and we would be left with the system that could never be trusted to produce justice.