a) You are missing that the human error factor is still present in the automatisation itself. Only you don't send one person to prison by mistake for an offense, but maybe a lot of people for a class of offenses. "Ooops, off-by-one error in Judge 1.1, 5 innocent people executed. Patch forthcoming."

b) One cannot encode laws in programs; a judge's task is to interpret the meaning of the words and apply them to a case. This is rarely done in local courts (which mostly really just follow a schema), but higher courts, at least in functional democracies, do actually look at the circumstances under which a law was created.