[fixermark]

Inconveniently, juries (and judges in non-criminal cases) do, in fact, make something legal or illegal by changing their minds. It is the function of a jury to decide questions of fact, because determining absolute objective truth in a courtroom scenario (from a scientific sense of the word) is often functionally impossible. So you're left with the situation where the plaintiff and defendant provide evidence that their respective versions of the truth are the objective reality, and the jury's subjective opinion of their case determines what the law agrees upon as objective.

This interface between objective and subjective is the "magic" of the legal system. If you can argue in a court of law, successfully enough to convince a jury (or judge, depending on the criminal / civil nature of the case), that, say for instance, a corporation is a person in the same sense as an individual human being, then as far as the law is concerned it is now an objective truth that corporations are people.

[boomshucka]

I'm not even talking about deciding questions of fact. If you take a set of facts as a given, it's still often not clear if something is legal or not. Legislation is often written in broad strokes and it is up to the courts to fill in the details. There are always new cases coming up that don't fit the same fact pattern as existing cases and hence no one is quite sure what the law says on such matter.