When Luffy awakened his fruit, the Five Elders talked about a previous government. They also do not behave like Celestial Dragons at all. Maybe they still are but Imu is not, if he is anything like Enel. In Skypiea, Enel had nothing to do with creating the conflict between Shandia and Skypieans. He just came and took over the political structure the Skypieans had build. Imu will be the same. The Celestial Dragons are also oppressed, folks. Luffy needs to free them! Maybe Imu took over 200 years ago. Some mysterious things happened 200 years ago.