Ohhh now I finally get it. Just saw the painted lady episode and I realize what they're trying to show. The fire nation is winning the war and they have all these horrible machines, all this technology. To make them, they have to destroy the environment in the process. The other tribes live in harmony with nature. But the Fire nation pollutes rivers and roads all in the name of conquest. Even their own people suffer as a result.
By the way it seems some of them are prejudiced, they scorned Katara because she was a water bender even though she helped them countless times. Of course they learned their lesson and helped each other in the end.