Civic combinations do not have to make sense in the world that we live in right now. The only "realism" that a mature fantasy world has to follow is for the reaction of its people to hypothetical civic combinations. For example, it is not "realistic" for a tundra world, but we can infer how a tundra world culture would be like by looking at how the people living in such areas in our own real world live, and then extend it to the entire planet.
That, and Calavante's example of "republic" and "slavery" in the Greek city states. Universal sufferage and slavery aren't that incompatible, unfortunately. You can vote segments of the population into slavery, because they aren't "legitimate voters". That's why I like the distinguishment between universal sufferage and emancipation in Civ4 regular, and the "unhapiness penalty" would only be granted to the emancipation civic.
Now that I think about it, maybe the liberty civic should be the one giving that unhapiness penalty instead. In addition, perhaps the liberty civic cannot be adopted by evil civs (therefore excluding sacrifice the weak).