I think Lore Reloaded's final conclusion makes some sense.
StarFleet itself cannot interfere with a civilization due to the prime directive, however the UFP Citizenry of it's Member Species/Worlds can expand and colonize as they see fit, especially with the scouting reports on new territory from StarFleet.
Then once the UFP citizens expand and colonize a new world, StarFleet itself must protect & support existing UFP citizens in their new colonies / annexed territories.
Regardless of the potential for any "Future Civilizations" that may spring forth from newly found planet, StarFleet might not be able to "Interfere" by it's own rules & regulations. But UFP Civilians can expand and colonize willy nilly.
Ergo vast expansion of the UFP without StarFleet getting their hands dirty.