Because doing an app well is hard. It's much much easier to wait until someone like Kicksend establish themselves, then give them some money and have them integrate.

And then there's the marketing: Walgreens would peddle the app to people who are in the store and thinking about printing digital photos. A Kicksend user might never set foot in Walgreens, but if he knows about the feature in the app (it's pretty discoverable), he might decide to print a few photos to stick on the empty wall next to his desk.