As far as we know, they showed no indication of staying on their home planet and not venturing out if they would have warp drive (and hence remaining unaware of the interstellar community), but of reacting negatively (or suffering a severe impact on their culture, take your pick) once they would find out they weren't alone in the universe.
Yup - I meant Malcorians
at the conclusion of the episode, with the leadership finally agreeing that stalling with contact would be in their national interests.
That state of affairs would remain as long as that particular leadership remained in office, and perhaps perpetuated by the following ones. Or then rudely cut by outside interference. But from the UFP point of view, the Malcorians would be a society disinterested in contact despite theoretically possessing warp, and posing political and monitoring problems specific to that setup.
But they didn't do that with the Edo. They weren't even aware of their basic laws regarding ball games
But they were - Yar did study the laws and was explicit that they were pretty standard (no doubt also including "don't step on the lawn, at least not in a way that creates lots of shrapnel"). What she missed was the chapter on punishments, apparently unwritten.
Observation would not have revealed such things any time soon, as people on the planet just plain obeyed the laws (or got lucky with the punishment zone thing) and the death penalty for jaywalking never would have become an issue in evidence.
I see no evidence that they covertly investigate a planet before exploring it at all.
Not within the confines of the episode, no. They interact with the society openly and directly. But they could do so if the planet had been previously investigated - and since they were founding a colony next door, odds are actually high that such investigation did take place. And it, too, would have missed the death penalty thing because of the extremely low statistical odds of spotting it in action.
Which is why I always assumed they simply avoided contact with any pre-warp civilisation.
In TOS, they clearly didn't. When they bargained for resources, there might have been long history of interaction, and all sorts of stories of early "avoidance" and later "pressing circumstances" or whatever. But we also saw episodes where the heroes simply waltzed in on worlds not previously contacted, on missions we could call "aggressive recce". These inevitably
led to contact, but they could still have been Kirk's ham-fisted version of "covert preliminary investigation".
The TNG setup of the PD not applying to non-Starfleeters is actually the most consistent scenario: all sorts of pre-warp, pre-radio, even pre-fire cultures could and would come under alien interference, Starfleet having no resources to stop that from happening, and the Feds would deal with that the best they can. But the PD would keep the Federation's most dangerous individuals, starship commanders, from becoming fearsome criminals dabbling in interstellar evil...
Timo Saloniemi