The Federation didn't even exist until, what, 'Devil in the Dark'? The consistency of TOS is something only visible in hindsight.
That's because the Klingons and the Romulans were cultures capable of interacting with the Federation on equal footing. A pre-contact, not-warp-capable species is not. Same with the restrictions on Federation usage of the cloaking device—a limitation that fits perfectly well with the Federation's purpose as a non-aggressive polity based around mutual consent instead of an imperialistic power.
The Angelites were already aware of alien civilisations (probably due to previous cultural contamination) and thus covered under the grandfather clause that covered Neural. We can only look at these two examples and derive the idea that allowed contact for grandfathered worlds depends on their level of technical knowledge and previous awareness.TNG even stated that the PD doesn't apply to civilians, whereas in TOS it most certainly did.
For counter-example, look at the researchers on Mintaka III—they're civilians, yet even more egregiously concerned about lack of contact than with the Neuralites.
Putting aside that they were written by the same people, VOY continues TNG and DS9's interpretation of the PD applying as a general principle of non-aggression and non-interference even with postwarp cultures. (see 'Caretaker,' 'Thirty Days.')After the egregious errors in TNG (IMNSHO the poorest of the Trek shows) people still have the temerity to pick on Voyager and Enterprise, which were both head-and-shoulders above the level of TNG.