Because both English and Russian speakers recognize the underlying principles of negation and affirmation. The specifics are irrelevant, otherwise English speakers and Russian speakers would be unable to communicate the notions of affirmation and negation, and you would be unable in principle to pose your problem.
So the law of grammar that says that a double negation is an affirmation is not a universal law, although it is a law of English grammar. So did God give us the laws of English grammar?