This picking apart the pre-warp terminology is quite unneccessary. The term pre-warp is used because warp is pretty common. The bottom line is, the Federation does not want to start being the cause of failed upbringings of a people before such time that they would bring themselves into a position where contact with other species is inevitable. Fight for your country, your patriotism, your religion and your belief system all you want but once you put yourself into the interstellar scale, you instantly implant yourself into the chaotic twists and turns of intergalactic politics.
As with the sub-light race, if they're trying to pillage a federation planet, it's hard not to make contact. If they are starting a colony on another world previously unowned by the Federation, write them both off as untouchable for a while.
At this point, with as many governing powers exist throughout the quadrant, a Nazi world would have a pretty tough time as an upstart to gain any hold in the stars anyhow.
One sure-fire way to be able to interfere in the affairs of the planet is have the Hitler-figure be a known alien to the world, interfering himself.