This was not a particularly constructive example to use in the original post, for several reasons.
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.
As the latter tends to be more likely, and in the former, there are still ways you can show interest without buying them a drink, you should not buy them a drink. However, buying them a drink is not wrong insofar as no unpleasant social consequences result from it (as they might result from, for example, an unflattering comment about a person's weight or appearance). All that happens is you're out $3-10, depending on the bar.
It's also worth noting that with certain people and in certain circumstances, you may actually be seeking someone with the qualities indicated by this request. If I were a rich and not particularly attractive older man, and the subject were a much-younger and attractive woman, this comment may actually suggest we could establish a mutually beneficial relationship. Our response to the request is really a response to the person making the request, and your hypothetical assumes we should a negative response, which is generally but not invariably true. Your description of the
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.
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: