One could characterize it as "manipulation," which has a negative connotation, but it could also be conceptualized as dealing with a person who brings something to the table—that is, who understands that your time / effort is presumably valuable, and that they shouldn't expect something for nothing. Trying to pay outright would be overly expensive and too intrusive, but a small thing like coffee demonstrates that the person wants to differentiate themselves from most people, and that they have something at stake.

People treat "free" as very different than "not free," which Dan Ariely describes in Predictably Irrational, and people treat exchanges in a special way, as Lewis Hyde describes in The Gift: Creativity and the Artist in the Modern World.

Manipulation: gift + nothing meaningful to share

Kind gesture: gift + something really meaningful to network about

(Or is that easier to do than I think?)

I forgot to add the 3rd benefit to the brewery is that I would assume, in general, when people stop in, they don't come alone, and they don't just have one beer and leave. Once they're in the door, they tend to spend more money.

But how easy is it to tie into twitter and a payment system? That'd be the cost.

Starbucks can distribute such costs (which may not be fixed, but certainly won't grow linearly) across a larger number of stores than your local brewhouse can.

The advantage over Skype with this use case is that you don't need to add them as a contact (especially if it turns out to be someone you decide you don't want to stay in contact with).

Disclaimer: I work for vLine.