There is also an implied contract with most requests. Many people if asked to buy a stranger a drink will assume that agreeing to the request will result in an opportunity for conversation at least. If someone makes the request with an understanding of the implied trade and no intention of fulfilling their half of the bargain then that seems at least dishonest if not actually immoral.
I wouldn't request a favor like this from someone I didn't plan to have at least a short conversation with. (I would ask smaller favors, like that they tell me the time, or more urgent favors, like that they loan me their cell phone so I can call my ride, but a drink is neither negligible nor particularly important.)
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: