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.

The simple fact is, the Federation is just like any other culture: if it doesn't expand, it will die. The difference is, they do it peacefully. Which they are entitled to do.