The Federation's highest laws, and this dates back to Kirk in TOS at least, states that "your world is yours".
But in Friday's Child, Kirk was talking to an indigenous people, about their own planet. If the Capellans were only 600 people from another planet, would Kirk have said the same? Unlikely.
If those 600 people were there for a long time (fact), and the colony was established by them (another fact), actually very likely.
How many times has the Enterprise-D visited an established colony, regardless of size or longevity, and respected the sovereignty of that colony? I think I lost count of that one.
Still, it is factually established as an ideal of the Federation, unless Kirk lied.
It is one of the key things that Kirk believes sets them apart from the Klingons.