The Southern Tribe still considers themselves entirely independent from the Northern on many issues (if Northern Water Tribe still has the "women stay in the healing" bit on bending, so it wouldn't be accurate representation for the water part if both had to vote for the same position. Too be honest, I don't think Earth Kingdom and Fire Nation (which probably is a rust belt since Republic City seems to be the king when it comes to manufacturing) have any real bad deals. Although, if anyone got screwed, it seems the Non-bending community doesn't have a rep on the council's decisions at all, so they don't exactly have a voice in the city unless it's through the Equalists and Amon.