Doesn't Earth's United Nations have some overtly hostile members at times, who make it clear that the UN should keep out of their business?
Certainly. But if we use this standard, we should argue that the Klingon Empire is a UFP member (quite regardless of the remarks made in the first season of TNG), and that the Cardassian Union is, and that the Ferengi Alliance is, and so forth...
In "Dax", we hear for example the following:
Bad guy Tandro: "The relevant treaty between Klaestron IV and your Federation allows for unilateral extradition."
How much clearer could the writers be about wanting Klaestron IV to be a non-member?