It's probably not strictly naivete - just keeping it simple and clear for visual displays etc.
But it's a good question.
And what would happen if a planet 'within' Federation space reached warp flight and decided they didn't want to be associated or involved with the Federation.
Or a non-aligned planet within wanted to create it's own alliances with another non-aligned planet.
Galactic politics becomes messy very quickly!
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist