Christopher...your interpretation of the Preservers is interesting and there may, in fact be a good reason to view them as some sort of interstellar Greenpeace. I can think of at least two other examples of aliens hauling Terrans away for their own purposes. We see it in Voyager's "37's" and Enterprise's "North Star." One might postulate that there was a time in Trek's galactic history where it was common for alien races to simply transplant small numbers of aliens to other planets for their own purposes (rather than conquer the entire world). We also know that some races directly interfered in human development, from the alleged Greek gods down to the Q. Hell, even the Vulcans studied Earth on several occasions.
The Preservers need not have even been a species, but an organization who visited worlds who's people were in danger of contamination from aliens and merely created cultural preserves where they could exist and develop free of alien intervention. Sort of a more proactive version of the Prime Directive. Its worth noting that the Preservers picked a Terran culture that would not have interacted with Apollo's people and would thus be potentially free of any alien influence.