The optimal play and the "rational" play are two different things. I've already stated that I would be very unlikely to offer a low offer to my opponent, mainly because I figure he'll be an idiot and overvalue spite.
[Emphasis mine]
Your terminology suggests you're seeing this as a game that you play to maximize your score. The very title of the thread supports that framing. If that was in the "social contract" of the interaction (as it is in a game of poker). I was intending this more as a metaphor for social interactions in the wild.
I use the word "play" in many contexts. Do not read into it as just a game.
What morally wrong act has the splitter committed in the original case?
Both players have equal claim (or lack thereof) on the money. There's no moral basis for anything but an even split, which is just a sane and socially ingrained default. As he deviates from an even split, the splitter not only gets utility at the boolean's expense, but he causes more harm than value he gets (because of the added grief of abuse). By increasing the risk of a veto, this disequilibrium further reduces the global expected value.
You may contest that there are "irrational" forces at play here, like the expectation of an equal return as the basis of what is "fair" in absence of other claims, and the drive to punish exploitive behavior. The conviction with which you call these irrational and idiotic suggests that you hold a reductionistic belief that game theory is straightforwardly applicable to (even stylized) real life interactions, despite assumptions like unbounded "rationality" on all parts, perfect information, perfect selfishness of actors, constrained time domain, isolated strategy space.
Don't you acknowledge the possibility that these "irrational" assumptions and biases may be evolutionarily stable, advantageous features for individuals and societies?
You
do acknowledge that if you were the splitter, you'd be more generous than you think you "should", in anticipation for "idiocy". Of course, it's the prevalence of such actual "idiocy" that makes it a credible threat. And that, in turn, gets "idiots" a better payoff in this game. So isn't this "smart", in a way?
There's no moral basis for an even split either. It just appears that way since the question is worded as an equal claim. But there is not an equal claim, as both players have different rules they must follow. If they were just told to take the money from a pile, and if they both could reject back and forth until they both agreed, and there was parity, it could be said they have equal claim. But the rules specifically make it so they are unequal participants.
I pretty much already stated your last point, that this game is a very special case and not something likely to happen in normal life. In normal life, especially during most of human evolution, people interacted with small groups on a repeated basis. Reputation was (and still is), a HUGE part of life. If someone is seen as generous, he will get more generosity from his neighbors. Those who follow through with generosity will get even more in return. By having that as the "default" interaction people have, it allowed people to trust each other, become more successful, and grow society. If you played this game in multiple rounds, you would see a much more fair outcome go forward. Punishing someone is not just spiteful, but a teaching lesson where you actually have the chance to benefit. If I punish someone by giving me a small reward, next time, they may give me a bigger amount. The game shifts more toward the person who stops things, who will play hardball more initially. Early rounds he may even reject fair offers to try to show he means business, and then the first person will give up and start offering very large amounts. Depending on how many rounds there are, the first person may choose to play hardball back, and refuse to offer anything too much. Eventually it will swing to 50-50, which is what you'd expect in normal human interactions.
The key here is the situation deviates from typical human interactions and situations. The trouble is, normal everyday life now is considerably different than it was 20,000 years ago. But we haven't really evolved, We're still the same hairless apes of 100,000 years ago. To get beyond that, we need to use our brains instead of our emotions. It was the few people who actually are capable and actually chose to use their brain that are the reason we are arguing on the internet instead of sitting in caves picking bugs out of each others heads. It certainly wasn't the emotional reactions that got us past that point.