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?