Like I said, warp drive isn't always a sign of cultural maturity. It just marks a point wherein contact is now inevitable so the Feds figure they may as well make it as peaceful as possible.

Exactly, and that's all there really is to it, IMO.
Anything else would be guidelines for those worlds who want actual admission into the Federation. I'm sure there are a lot of starfaring societies out there who aren't even close to meeting the Federation's criteria for membership (nor want to join anyway) but are recognized as sovereign nations in their own right.