Actually "Pre-Warp/No knowledge of spaceflight" WAS the main and only condition of the PD in the TOS era.
No, in fact, that provision
was never explicitly stated in TOS. (
See also)
From "Return of the Archons":
SPOCK: Captain, our Prime Directive of non-interference.
KIRK: That refers to a living, growing culture. Do you think this one is?
From "Bread and Circuses":
True, that does mention not giving knowledge of other worlds to a society that lacks it, but that's not about warp drive, just about having the
knowledge of alien life. You don't need warp drive to know there are aliens -- see the Capellans, the Eminians, and various others. It wasn't until TNG that "unaware of other worlds and species" was glossed into "pre-warp." Indeed, it wasn't until "First Contact" in TNG's fourth season that the invention of warp drive was defined as the qualification for contact.
But "The Magicks of Megas-tu" makes it explicit that the Prime Directive is
not limited to pre-warp cultures:
Look, look at General Order number One. No starship may interfere with the normal development of anyalien life or society.
"
Any alien life." Nothing about warp capability.
TNG started applying the PD to "any non-member Federation world" which was ridiculous.
Of course it isn't ridiculous. Warp drive has jack-all to do with it. As Greg said, the PD was meant as a counter to cultural imperialism and colonialism, the idea that one society had the right to impose its will on others "for their own good." History had shown the damage caused by colonialism and coercive religious conversion, not only in dealings with less advanced cultures, but in meddling with equally advanced cultures, like the damage that British imperialism did in the Middle East, India, and China. (Not to mention that, contrary to colonialist myth, the Native Americans were comparable in advancement to the early European settlers, just specializing in different technologies and methods.)
So the Prime Directive was
never meant to be anything as shallow as "Psst, hey, guys, don't tell them there's life on other worlds." The point is to prohibit Starfleet officers from forcing their beliefs and values on any other society, because history has shown how disastrous that is. It's about respecting other cultures' right to make their own choices, and it's absurd to say that right somehow ceases to exist on the day a culture invents warp drive. It just means that, once they reach that stage, the parameters are different. You don't reveal yourself to pre-contact worlds because they wouldn't be able to compete on a level playing field and could be easily overwhelmed, so the risk of disrupting their cultures is too great (or so the theory goes). But it's still possible to disrupt warp-capable cultures, the same way that Earth nations have disrupted each other for millennia, by cultural or military imperialism or economic manipulation or religious oppression. The Prime Directive is the rule that keeps the Federation from thinking it's entitled to tell other people how to live. And that means "Don't force the Klingons to give up being warriors" and "Don't take sides in somebody else's civil war" just as much as it means "Don't force a world to discover alien life before it can figure that out on its own."