Interesting. I had a similar idea, but I think the UFP comes out in a better light. Look at it this way: All those pre-warp civilizations in Federation space would certainly be contacted prematurely by other Alpha Quadrent Powers who do not have an equivalent to a Prime Directive. In order to enforce that Directive, the UFP has to declare interstellar rights to those planets to protect them. It seems that they then leave them alone or just observe until they prepare for their first warp test. Then the Federation makes official First Contact and tends to treat the worlds as soverign nations unless they pention for admission and join as UFP members. So although there are similar parallels, I think there are important differences from the colonial expansion of Earth's past.