You mis-understood. When I speak of the self-determination of a country, I'm talking about the country's right to follow its own path of development, not the right (or lack thereof) of the individuals within the country to determine the form of its government.
Real life isn't "Star Trek", the "Prime Directive" is just a fabrication of some sci-fi writers, not a real moral code. There is no inherent right of a government to oppress its own people, at least not based upon any rational moral ethic.
For example, a country's history may lead it to becoming a tyrannical theocracy through sheer weight of history rather popular consent of its people. That form of government is a product of that country's particular background and current situation. They are what they are.
Isn't this just a rationalization for isolationism? No matter what happens "elsewhere" it's none of our business?
I don't buy that and I don't think you do either. Oppression, be it gender, ethnic, political or religious oppression is the business of everyone in every part of the world. The only real issue is what we're willing to do to enact change. The spectrum is everything from mild public criticism to war for regime change.
As long as it stays within their own borders, our only influence should be in terms of how we interact with them economically, politically, and socially (I suppose). In essence, if we want to be the bike-riding Jahova's Witnesses that knock on the door and ask, "Have you found democracy?" That's fine. That country still has a choice about whether or not they want to "find democracy", but the moment we kick in the door, we take away the choice that we fear will be taken away from us.
You don't really believe in a "right" of a government to oppress its own people (if it happens to be "its own path of development"), you just want to take war off the table as an acceptable method to enact change.
I certainly agree that other methods should be tried first, but if you try to rationalize opposition to war by trying to create a new moral principle that other governments have some sort of "right" to do whatever they happen to be doing, you end up rationalizing apathy and isolationism, and that's wrong too.
Hundreds of years ago it was believed rulers ruled by divine right, that everything they did was God’s will, and the evidence for that was that God put them in power to begin with. That sort of thinking rationalized all kinds of abuses.
This is just a reflection of that thinking, only it replaces God with an arbitrary “path of development.”