The Atlantis fleet came to Earth and gained the approval of various countries' governments. As they did so, the first thing they admit to the general public was acknowledging their ancestor's mistake of having a forceful fierce conflict with Earth. They recalled having such a feud to seal the foreign world.
Then, they also expressed that to maintain the 'purity' of the universe, they would spare no efforts in helping the Earthlings to defend against the invasion of the foreign worlds. The high—almost perfect—moral standards that the higher civilizations displayed had gained a high admiration from most Earthlings.
However, how could a powerful interstellar civilization with billions of people not have countless dirty 'scum' and complex social relations under its glamorous appearance? A smart person would know that every Atlanteans couldn't have a heart as beautiful as their appearance.