There are basically two situations that lead to this: First, the other person is interested in you, but is somewhat awkward and uses this as a rather blunt test to measure your interest in them. Second, if person is not interested in you, but sees you as a means of getting a free drink.
That doesn't cover Mallah's story. I think the free drinks explanation is largely a confabulation by girls who don't know why they do it.
It covers the story up to the point of her not taking the drink. Perhaps she just wanted to see if she could. I agree that getting him to buy the drink may be more significant than actually consuming it. Or it could simply be a way of chastising someone she didn't believe should be talking to her. Or, perhaps, she simply forgot.
For practical purposes, she's not different whether she takes the drink or not. It's still a waste of money. If she didn't forget, it's likely she simply got a kick out of ordering some guy around. Not a situation I anticipated, but certainly deserving the same response as the selfish drinker.
Followup to: Do you have High-Functioning Asperger's Syndrome?
LW reader Madbadger uses the metaphor of a GPU and a CPU in a desktop system to think about people with Asperger's Syndrome: general intelligence is like a CPU, being universal but only mediocre at any particular task, whereas the "social coprocessor" brainware in a Neurotypical brain is like a GPU: highly specialized but great at what it does. Neurotypical people are like computers with measly Pentium IV processors, but expensive Radeon HD 4890 GPUs. A High-functioning AS person is an Intel Core i7 Extreme Edition - with on-board graphics!
This analogy also covers the spectrum view of social/empathic abilities, you can think about having a weaker social coprocessor than average if you have some of the tendencies of AS but not others. You can even think of your score on the AQ Test as being like the Tom's Hardware Rating of your Coprocessor. (Lower numbers are better!).
If you lack that powerful social coprocessor, what can you do? Well, you'll have to run your social interactions "in software", i.e. explicitly reason through the complex human social game that most people play without ever really understanding. There are several tricks that a High-functioning AS person can use in this situation: