Yeah, humans are often shown as being permitted to leave the Federation and found colonies of their own. This is one of those cases.
Ok, I think we're beginning to narrow down Gene Roddenberry's optimistic vision of a future where humanity is perfect.
1. Humanity is perfect only as children.
Children do not grieve over the loss of loved one even though the adults do ("The Bonding") and their intelligence is so astronomically high that they can do in minutes what an adult chief engineer tells him would take weeks even while drunk ("The Naked Now").
2. Humanity is perfect as long as they are on Earth.
Gene Roddenberry: Humanity is perfect and Earth is a Paradise.
Sisko: "On Earth there is no poverty, no crime, no war. You look out the window of Starfleet Headquarters and you see paradise. "
3. Humanity is perfect as long as they remain in the Federation.
Any planet consisting of human colonies that is not within Federation jurisdiction will all eventually turn to complete and utter anarchy, resorting to waring and raping methods of living on Turkana IV to the other colonies (including human ones) that would eventually form the Maquis.
Human children between the ages 5 and 17 who live on Earth and are a part of the Federation represent the absolute perfection that humanity has to offer. After they hit adulthood, they are no longer perfect for they will start caring about the lives of others, feeling grief and wanting to set out and explore the unknown. I guess the children at Little Lamplight were actually on to something and the isolated and suicidal society of Logan's Run is the best course of action to ensure a perfect humanity, so long as the age is reduced from 30 to 18.