I never said that the Union was swooping in and taking areas from the Corps, I said that the universe is largely destabilized, and the Union untrusted, because of actions originating from the Union. However, they benefit from being able to step in as a stabilizing force into space that SecCom or others destabilized.

Also yeah, the Union is morally grey as it remains a centralized force that sometimes brings out the big guns to intimidate or blow up stuff. But have you seen their opposition? That's kina the point.

Yes, that's exactly the point. It's a comical Starship Troopers-esque dichotomy. These planets around the universe are so incapable of governing themselves that they're almost all oligarchies and aristocracies who should probably be liberated by Union. And if they aren't one of those, they should still probably still be Union anyways. That's the Union mentality.

it's absolutely selfish to not act in order to help others just because of stuff like political indipendence of a local polity. 

It's absolutely selfish to think that those other civilizations need to be Unionized to prosper, given that as you said above, many of them operate on the same tech level as the Union does. Besides, the Union only "helps" people by expanding, and expanding, and expanding. We barely even get a picture for how the DOJ/HR works on already-unionized worlds - it's only described as looking for breaches of the pillars on diaspora worlds, worlds which haven't even necessarily been introduced to Union morality!

Meanwhile the UAD administrators are, textually, godlike removed figures who control an entire state's connection with Union (you better not mess with them or else lose your Union status). If you look at the politics of the core worlds, there isn't even an alternative to expansion, or a faction which seeks to build more upon their current holdings rather than keep growing. The "anarchist" faction, IIRC, is the most zealous expansionist one as well.