They are banned in the Hegemony too and that didnt stop them asking for a gamma in the tutorial.
And tbf the League is stated to be a loose grouping of planets with no real central government aside from a vague 'we hate the hegemony' common idea, so that one of them is less opposed to cores is not impossible.
And yeah, we dont see any 'restarted' terraforming, but then again, I get the feeling we arrive in the game when the sector is finally stabilizing to some degree, and that previously its all been chaotic and not really lending itself for big civilian projects like terraforming.
Otherwise it makes little sense why no faction has explored the ruins and stations we find, why no one has colonized nice world right next to the core we sometimes find (Even in Duzhak), or why we, a single person, can foster a planet/system to become larger and more powerful than the capital systems.
Either we are 'super special player' or the factions have been so busy fighting and making sure their current worlds dont crash and burn to do much so far and its now that there's even the consideration of looking past the core worlds.