star trek - Why does the Prime Directive not apply to Rubicun III?

In a first season episode of Star Trek: The Next Generation the Enterprise visits a planet named Rubicun III where (according to Memory Alpha) the Edo live.

It is often mentioned in Star Trek that the Prime Directive requires that pre-Warp civilisations be ignored until they discover Warp mechanics. But this planet and civilisation appears to be exempt from that status. Why?

Answer

Criticism about the effective application of the Prime Directive in Star Trek stories goes back to The Original Series and every iteration has a few episodes that gloss over, overlook, forget, or simply ignore the Prime Directive for the sake of story continuity.

In the case of the Next Generation episode Justice, in which the Edo appear Wikipedia indicates:

In contrast, the Next Generation episode "Justice" did not explicitly explain whether the Edo people were pre-warp or were aware of offworld space travelers prior to the Enterprise's visit. If the case is the former, then when Wesley Crusher is sentenced to death, the violation of the Prime Directive had already occurred and the issue of rescuing him, while politically exacerbating matters, might not have been a violation of the Directive.

Emphasis is mine.

Violations of the Prime Directive are numerous and recounted in the Prime Directive entry in Wikipedia:

One criticism regarding the Prime Directive is that it is inconsistently applied, depending on a planet's strategic importance or the circumstances in which a starship crew finds itself. For example, as part of the Federation's then-ongoing hostilities with the Klingons, Captain Kirk was ordered to make contact with the seemingly pre-industrial Organians in "Errand of Mercy." In addition, Kirk directly interfered with the laws or customs of alien worlds in "Friday's Child," "For the World Is Hollow and I Have Touched the Sky," "The Cloud Minders," "The Apple," "The Return of the Archons," and "A Taste of Armageddon," in order to achieve a Federation objective, to save the lives of his crew, or both.

Compounding matters is that in the TOS episode "The Omega Glory," Kirk states, "A star captain's most solemn oath is that he will give his life, even his entire crew, rather than violate the Prime Directive," and yet he seemingly violates the Prime Directive as "the only way to save my ship" in "A Taste of Armageddon" and no explanation for the Federation Ambassador trying to mediate between Eminiar VII and Vendikar (neither of which are Federation members) regardless of their wishes on the matter is given.

In "The Return of the Archons" and "The Apple" reference to the "Prime Directive of non-interference" is made by Spock. In "The Return Of The Archons," Kirk says the Prime Directive refers to "a living, growing culture" to justify interfering with what he sees as the non-development of the computer-controlled culture, asking pointedly in reference to it, by contrasting it with living, growing cultures, "Do you think this one is?" In the "The Apple" Spock points out that Starfleet Command may not agree with his choice to interfere with the computer controlled culture to which Kirk replies "I'll take my chances."

There are also episodes in which the Prime Directive should have been mentioned, but wasn't. In "The Paradise Syndrome"," the Enterprise attempts to save a pre-industrial planet by moving an asteroid that was on a collision course with it; when McCoy asks Kirk if he should warn the people, Kirk and Spock only point out the people would not understand the warning, and neither makes any reference to the Prime Directive. In "The Cloud Minders Kirk interferes with the culture of Ardana to obtain zenite the only cure for a biological plague ravaging Merak.

Admiral Matthew Dougherty's reasons for violation of the Prime Directive in Star Trek: Insurrection in Picard's time echoes the reasons Kirk gives McCoy in "Private Little War" but Picard considers them invalid. In "Homeward," Nikolai Rozhenko (Paul Sorvino) uses holodeck technology to save the Boraalan and enforce what he believes is the spirit of the Prime Directive even though Picard has already said such actions violate what it actually states. In "Pen Pals," Captain Picard rectifies contact with an inhabitant of a pre-warp planet by ordering her memory erased. When contamination became too serious to be fixed by memory erasures, Captain Picard decided to make direct contact with a civilization's leaders in "Who Watches the Watchers" and "First Contact," although the latter episode involved a planet on the verge of achieving warp flight, and therefore eligible for First Contact. Finally, in "Natural Law," the Voyager crew took measures to ensure the protected isolation of a primitive people, even from a more advanced civilization who share the same planet.