The jury is an old form of democracy. The king/government can make what laws they like, but it needs to have enough popular support that a jury will freely choose to convict according to it.

Of course, like all forms of democracy, the outcomes don't always agree with your or my sense of justice.

> it has to be balanced against crimes that are in principle clearly wrong

In this system, whoever gets to decide what is "clearly wrong" has the real power.