Luckily Civ3 is not the real life so we can ignore all realistic details that would ruin the game or otherwise be dull and uninteresting.
We don't need to be slaves to realism here. Basically the idea is to have a political arena in the game where the civs are provided tools and opportunities for intrigue, power grabbing, warmongering, bullying, backstabbing and horse trading. In the real world the UN charter is to promote peace. In real life peace is also nice whereas in a civ game it's just dull. Conflict is the magic word.
(Now, whether the real life UN charter matches with the actual reality is completely irrelevant here, so let's leave it at that. There are better forums to discuss politics.)
Game fact: If every civ... heck even if just one or two civs had a veto to UN edicts in the game then the whole idea would be completely moot. In Civ3 the civs are out for themselves and to the devil with the others. Veto can't work, not in the game as a game option. Therefore we just calmly leave it out: no veto for anyone. That way things actually can get interesting. Which is the whole point.
Besides, the rule that a proposed edict has to be seconded by another civ is kind of a veto. It ensures that even if a big super-civ controls more than 50% of the votes it still can't force its edicts through without being nice to at least one other civ. Which would also be good from the game balance point of view.
Another game fact: If every civ had an equal vote it'd also get dull. Smaller civs consistently ganging up against the biggest gets tired after a while. Answer: so don't make them equal. Relate the voting power of a civ to its actual power in the game. Heck, this would even be realistic as it represented the ability of more powerful nations to persuade, bribe and bully weaker nations to support it.
You just have to stop thinking of UN as a benevolent international organization for All Good And Nice Things and instead view it as another weapon of mass destruction in your civ's arsenal.
