Ever since Starbucks hit it big with their timely Ray Charles CD -- which was certainly aided by his death and by the hype surrounding the biopic, Ray -- there has been a frenzy of speculation about the coffee chain's sudden move into the music-retailing business. Who do they think they are, anyway? Starbucks doesn't announce how much they make through their music-retailing, and they admit that it has more to do with a general corporate strategy than making a quick buck. In a Wall Street Journal article, Starbucks' CEO explained that he hopes to make a Starbucks "third place" behind home and office in the everyday lives of its customers.
Because customers are in everyday, and because Starbucks will concentrate on a pimping only a very few CDs at a time, the company can familiarize its patrons with a single CD and hope to hook them by the end of a week or two. Starbucks, then, serves as a radical filter, allowing its customers, who probably don't typically care much about keeping up with music, to simply have an "up-or-down vote" on a specific record they are slowly getting to know over time. The Pavlovian association of the music with the addictive stimulant probably helps to over time establish a link that makes that up vote that much easier.
Labeling music as appropriate for coffeeshops, as music to relax Starbucks patrons, has been a common trope in pop criticism probably since Starbucks went national, probably because there's such an obvious synergy there, and an obvious opportunity to drive music further into the background and make it aural massage. Starbucks has assiduously cultivated an aura of upper-middle-class hauteur (which is why "latte-sipping liberal" entered the lexicon so quickly, too) and its success with that means they have an imprimatur to extend to whatever leisure product they wish to find space for. Starbucks wants to sell a lifestyle, a self-image, not coffee, and music is the lifestyling commodity par excellence. It serves no function otehr than to advertise a sense of who you think you are, and what kind of chilled-out groove you want to asociate yourself with.