It is though, isn't it? I mean, not directly, but nothing can be translated directly like that. You're assuming the other person agrees with your sentiment. When the person says they're going to eat sushi, and you say good, you already know that they agree that it is good, as they were the one that said they were going to eat it. It's a common stance between you and the conversation partner.
For the third example, I'd still say that person is replying yes as a common agreement. I guess I should say that the rule isn't necessarily expecting the other person to agree, but more agreeing on a common stance. "Sou desu ne" to me means "it is (I agree/isn't it?)". But again, I'm just starting to learn, so maybe my understanding is completely left field. But in the case of say, "Ii otenki desu", replying with "sou desu ne" to me just means "yes I agree on the situation" rather than a literal translation of "it is, isn't it?".
I will say that my time in Tokyo made me think this as well, as a few Japanese friends I had would repeat "ne" while I was talking, almost like "isn't it?" would be repeated in US conversation.