I think the disagreement boils down to what different people find would be a fair split of the money.
I don't think there is much disagreement that if you had a good reason to spite the other guy, spending some small amount of money to pay back would be warranted. tomcollins admitted as much in his reply to the (admittedly extreme!) question of vetoing the guy who killed his family.
So the real question is: should you have any reason to spite the other guy? Which comes down to: is he being an asshole by asking for 99.998% of the money, or is it fair for him to do so just because he "can"?
There is a mindset --let's call it carebear; just a label, no ridicule intended-- that expects humans to display some sensitivity towards each other's utilities. We are social animals endowed with empathy. Morals are almost innate, common sense rules of thumb that make life better for everyone. Equality is seen as a sane default, even in non egalitarian cultures. An inequality must be warranted: someone worked harder, was smarter, or got luckier.
For a carebear, the fair split in the Ultimatum Game is an equal one (althought others may be pragmatically accepted), and the Splitter is being a jerk for trying to abuse his position. He's failing the basic rule of "do as you'd like to be done to you". Being put in the position to accept $10 is doubly indignating. Not only is he trying to get almost all of the money, but he's relying on you being nice about it (the utility of the $10 themselves barely registers, in this context).
There is a mindset --let's call it cutthroat-- by which, at least when money is involved, humans are expected to behave as selfish aggressive maximizers in a game with only the most basic ground rules: basically, respect for physical integrity and property (some would argue that real cutthroats won't respect anything that can't be defended, but again, it's just a label, take it as defined here). For the cutthroat, morals are there mostly to avoid physical violence, and behaving morally consists of refraining from theft and aggression. Behaviors that a carebear would consider "abusive" and "exploitive" are not immoral unless mediated by violence or threat thereof.
For a --um-- throatcutter, the Splitter is just being logical so it's wrong to spite him in first place. The destructive spiteful reaction thus seems doubly irrational.
Of course I'm simplifying a bit. Barring that, any big objections so far?
I liked this post a lot. I don't think anyone has any moral duty to me other than not to steal my stuff/cause physical harm/defraud me. If my sister wins the lottery, she is under no moral duty to share any of it with me (although it might be nice). If I get myself into trouble, no one owes me by bailing me out. If they do, great. I do not consider the person dividing the money to have any moral duty to me to share anything. I would graciously accept anything he chooses to offer me.
The key definitely where you set the baseline of fairness and moral duty. The trick is to mislead the reader into thinking that the money should be equally split. By phrasing it as "they have money together to split" is a great way to trick the reader into thinking that each player has an equal claim, and taking any more is stealing. But it's not the case.
The logic of some people is truly frightening. If someone does something "foolish", they deserve punishment! I would hate to be a neighbor of such a person, where I might do something they consider foolish, and they retaliate by burning my house down or slashing my tires since I "deserve punishment". Or if I won the lottery or came across a large sum of money, if I did not share it to their liking, I would be worthy of their wrath. I would suggest a career in government where they would fit in with other thugs.