Living under fascism is not nor should ever be a choice.
At what cost?
At what point should the allies have appeased Hitler?
How much should NATO appeased Putin?
The Federation ended up back at war with the Cardassians anyway and at that point the Cardassians had the backing of a stronger that.
If the federation had come down hard on Cardassia from the start and fully dealt with the spoon heads in the border wars and kicked the military out of power and backed the civilian deppa council then the dominion would likely not have been able to use the Cardassians.
O and screw the prime directive when dealing with fascists.
Those colonists
chose to live on planets that were 'hotly contested' (Admiral Necheyev's words, which were accurate) decades ago. With Federation territory being as large as it is, and being established that there were a LOT of other worlds to pick for colonization, the colonists knew that something like this could happen later.
They were given options to go elsewhere that was
not under dispute, but they
chose to stay.
When things didn't go the way they wanted, they blamed the Federation for
THEIR OWN DECISIONS. That's called not taking responsibility for your decisions and actions. It's also a sense of entitlement, which goes hand in hand with not taking any responsibility.
While I agree the treaty was bad because of what happened later, the Federation is also not one to 'come down hard on Cardassia' and overthrow a government. Doing that makes them exactly like Cardassia... worse because they know better. You know the Cardassians basically took over the Bajoran government, right? Was that right of them? The Federation has no place or right to take over another power's government, whatever the reason.
Had the Federation done what you said, the population of Cardassia would look on the Federation even worse because they
forced their ideals onto their government. The resentment would be even deeper and Cardassia would have become even more hard lined against the Federation, and instead of just one narcissist speaking for Cardassia and taking control of the government and allying with the Dominion, you'd have possibly the
entire Cardassian government ready to get in bed with the Dominion, and the resistance we saw at the end would not have happened and very likely would have cost the Federation and its allies the war.
The people of Cardassia saw how bad their system was on their own, and were already working toward taking out the military's influence. You can show people stuff but they have to come to these conclusions on their own. You know the phrase about leading a horse to water... applies here, too.