I used to work at Starbucks (albeit going on 5 years ago) and although I don't have the exact statistics or a perfect memory, it was not a "vast majority". If I had to guess, I'd say probably something like 2/3. Maybe the ratio increases if you count Starbucks gift cards.

Either way you look at it, Square's transaction volume surges with this deal. Good on both parties.

You don't enter a PIN? That process easily makes it take longer than cash.

I've read before[0] that it is actually a violation of a stores terms with Visa to enforce minimum transaction sizes. Those stores could potentially be reported to Visa and have sanctions placed on them (like an increased transaction rate). Interestingly, a merchant is allowed to offer a discount to people who pay by cash but they can't charge more for people paying by credit card.

[0] - http://www.msnbc.msn.com/id/11697094/ns/business-consumer_ne...

Starbucks don't.