There have been episodes of TNG that showed Wesley testing to see if he would get accepted. Picard mentioned that he failed his entrance exam the first time. So it seems there is some requirement beyond being a federation citizen for gaining acceptance.
Even Federation citizenship may not be required. Ensign Ro was Bajoran and Bajor wasn't a Federation member.
Or maybe Ro Laren was a Federation citizen even though Bajor wasn't in the Federation.