Regarding double negatives, I get what you are saying, but they absolutely can be interpreted as a positive - this is easily proven by simply reversing one of them, and they can be reversed because they are after all negatives.

But if the speaker’s meaning is clear then of course it’s rude and incorrect to misinterpret them.
I feel like there’s a gray area though where some constructions may be genuinely ambiguous which way the speaker meant (since a double negative as negative by definition means the opposite of what the words would mean otherwise)