The legal system in Westeros leaves much to be desired. There is no dedicated feudal analog of a judicial branch, ideally guided with fairness, objectivity, and the concept of justice. Instead, legal disputes are adjudicated by lords who often have vested interests in the outcomes and the overpowered ability to settle disputes by fiat (and with the martial support to have their decisions enforced.)
One would hope that disputes aren’t entirely decided arbitrarily by feudal lords; that local customs, precedents, and traditions might hold sway. But that’s not a given when the common-folk are facing the sharp end of Westeros justice.
Although the smallfolk of the Seven Kingdoms have less flexibility when it comes to facing legal issues, those with more status and privilege do have the option to take decision-making out of the hands of overlords and into their own. If they can accept the risks of Trial by Combat.