Since when the human legal/law system makes sure everyone understands law well in order to sentence? Do you mean that human legal system is in total failure? Get a clue!
There is a big difference between human legal law and "God's law".
Sure, most people don't know the full details of human legal laws. But in modern societies those laws are written down and it is clearly noted when they were written, when they were amended, and by whom. We can all refer directly to the laws if we want to, and there is no dispute as to what the laws state. The only dispute may be how the words are interpreted. But that's why we have judges.
"God's law" is pure hearsay. If God and his laws really exist, why doesn't he reveal himself properly, give each human a copy of his laws, and in the case of any dispute appear as a judge of absolute authority? If we humble human beings can work out things like that, why can't he? It's pretty obvious that even if he exists, he's an idiot and doesn't deserve any attention, let alone our obedience.