Really? It seems perfectly understandable to me. It's like Transformers, a bunch of fucking robot aliens drop down onto our planet and proceed to "save" our planet from the evil members of their race that are partly responsible for the destruction of their own homeworld and in doing so they end up destroying half of an American city. Sure, you might say "thanks" at the end of the day but a larger part of you would be like "Hey, you stupid fucks if you didn't come to our planet we wouldn't be caught up in your stupid interstellar civil war."
Just because Superman saved them from Zod doesn't get rid of the fact that Zod only came to Earth because Superman was there. Thus, Superman has inadvertently put Earth on the galactic map. Then there is also the fact that there is an all powerful super being who could totally kill everyone roaming around and who flat out said at the end of MoS "fuck you, US government trying to keep tabs on me" (paraphrasing), but promises to always be good. I'd be scared too.