Regardless, they're very clear on the post:
>Square will process Starbucks U.S. credit and debit card transactions, which will significantly expand Square’s scale and accelerate the benefits to businesses on the Square platform, especially small businesses, while reducing Starbucks payment processing costs;
Starbucks saves payment processing costs. That's benefitting them.
So for transactions less than $25 square comes out ahead. I can think of plenty of restaurants that are primarily lunch driven for whom square would make sense (not to mention coffee shops).
One interesting problem with this deal is that it will call attention to just how much money Square users really are spending at Starbucks.
Imagine if most or all of the charges on your Visa bill were from Starbucks, such that you couldn't help but see the total staring you in the face every month. That's the position a Square user will find himself/herself in, at least until more merchants come on board.