But did those planets agree to be encircled by Federation space or did the Federation decide it for them? Perhaps because at the time they were not space-faring? Or First Contact had yet to be made?
If such a planet pops up - and it doesn't decide to join, then who decides upon the borders of it's space?
In that case there should be agreed 'neutral' trade routes that do not come under any one powers' auspices and allows planets to conduct their business without the automatic nosiness of another looking into every ship that comes and goes.
If a non-member world is trading with the Romulans it's essentially none of the Federation's business without justifiable grounds it's anything of danger to them and it's simply intereference in the affairs of a sovereign planet if they do inspect all shipping.
Unless the Federation does it to make life as difficult as possible for the non-member in the hope they take the easier option and join up.
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist