The degree of comfort that people (considered as both individuals and collectives), exhibit with a given way of life is a tricky thing to examine in moral terms; even more so when we're dealing with alien species whose minds might work differently from ours, and whose psychological requirements might be distinct. First we have to take into account that education, tradition, and propaganda (didactic conditioning both "good" and "bad") will play their roles - very large roles - in shaping what a person and a culture considers normal, natural, defensible and moral. Community pressure, whether subtle and unacknowledged or knowingly reinforced, will be just as influential as natural instinct. Indeed, people will very often reinforce attitudes and behaviours in others for their own personal benefit without even realizing consciously that they're doing it. They can be selfish and exploitative, but without malice and without consciously intending to be exploitative at all; they might even be genuinely loving. This is difficult to deal with, as I'm sure others would agree. And then there's the eternal question - assuming we take note of what instinct rather than culture is telling us, which
natural instincts should we heed and which should be overcome or repressed? Civilization depends upon rejection of instinct...but it cannot assume that instinct is inherently bad, and must also be in some accordance with human (or Cardassian) instinct in order to feel "right".
Individual personality; natural urges; cultural and traditional teachings - how much can we trust in a given person's acceptance of their cultural or social norms when all these things are shaping them, and might be misleading them? Can we point to someone saying "Oh, it's alright with me" and use it to excuse or justify something that we think is unethical or harmful? To what degree can we trust what a given person says about how comfortable they are in their current lifestyle? Very difficult questions. After all, despite the image of "oppressed" people longing for liberation (most often used to justify third-party aggression or government manipulation, to be honest), in reality most people accept the norms of their culture however objectively harmful and restrictive those norms might be. Going against the group just isn't the way most humans work, and a great many will just accept what's normal without complaint, or at least without the urge or motive to cause trouble about it. Getting a group you think has the short end of the stick to accept it and challenge it can be just as difficult, if not sometimes more so, than changing the minds of groups or people you think of as having a better deal.
On top of that, even if we assumed that humans (all
humans) had a "correct" system that best fulfils their psychological needs and is most comfortable and beneficial to them (itself a problematic assertion in many ways) we must wonder if such a system can be projected onto another species. While I'm sure we'd all agree that the Cardassian Union has gone very wrong somewhere, and is detrimental to the Cardassian people in many ways, where we'd disagree perhaps is on whether their system has its merits and has just been corrupted, or is inherently
unworkable and crippling to their development. To what degree do you "work within the system" to change things, and to what degree do you assert that the system is inherently unworkable and must be thrown out entirely? And having made that decision, how do you justify the pain and harm that either approach will inflict on others? Because there will be victims and exploited either way.
Perhaps we have to ask ourselves whether Cardassia is misguided or just plain wrong. To give grossly oversimplified examples: Does strong patriotism lead inevitably to nationalism? Does a culture of military service lead inevitably to militarism, deindividualization and aggression? Does a strong centralized government inevitably mean tyranny?
Without going into details, I can think of several underlying assumptions about the way human societies near-universally work that I
consider unethical and, in the long-term, potentially disastrous for the instabilities they bring to our people. My perspective finds others' acceptance of those attitudes ("others" meaning both those I think suffer due to these norms and those who suffer less, for such distinctions as "wrongdoer/exploiter" and "victim" are so often largely meaningless when you get down to the really hard issues) - infuriating and even upsetting. I often think, It doesn't make it alright because you've been raised to think it's alright, and because those who convinced you of such were themselves fully convinced that it was.
But what can we do? All of us have to deal with these questions - which is why the OP is to be congratulated for raising the issue, if I may say so.
Another point is that change is often distressing; instincts tell us (and presumably also tell Cardassians and any other sapient races) that the familiar is preferable to the uncertain. It can often be hard to break out of ideological patterns even when they're actually maladaptive, or have become so; especially when there's pressure to conform. In the case of Cardassians, I think we have to remember that they've had a long, hard struggle against their natural environment. We don't know the details - what exactly they faced, what was mismanaged or taken advantage of, what was true desperation or necessity and what was incorrect assumption - but we know that Cardassians are a people who have long believed (been taught/assumed/known/take your pick) that in order to survive they must be harsh, strict, disciplined, self-sacrificing and dismissive of the individual if they want to secure the safety of the group (which they seem to value more). After 500 years of this, convincing them to change would inevitably be difficult. Natural instinct and centuries of tradition underlie a lifetime of education and conditioning, and all teach Cardassians (perhaps) that "this is how we simply must be".
And yet we know that there are Cardassians aplenty who, one way or another, question and challenge the way their people approach things, or seek to adapt further rather than stagnate. As with any society, it's akin to an ocean. From afar it looks conformist and calm; get closer and there are all sorts of choppy waves and currents. We are all adolescents, torn between the need to be with our parents and accept their ways, and to strike out independently for something more in keeping with who we
wish to be.