I just don't see what the experiment was about really considering the audience entirely consisted of people who can afford coffee - I just don't get it! The only explanation is a viral marketing scheme in my mind
If you read the post, he mentions examples where people buy someone else who can't afford coffee. That was what he encouraged from the initial post as well. Also I've seen many of my friends actually do something like that.
The latter makes me feel happy, and makes me want to be generous, too.