Of these two factors, the wants of customers is the more important factor in Starbucks' growth. However, it is likely that a segment of Starbucks' customer base wants to patronize a company that acts in ethical ways.
While Starbucks is now known for things like fair trade coffee, it was not always seen in that way. During its period of most explosive growth in the 1990s, its ethics were not its main selling point. Instead, its high quality coffee, along with its vibe as a cool coffee place from Seattle (which was a cool city in those times) were what attracted people to Starbucks.
While some people are attracted to Starbucks for its ethical stands, this is not what gave the company its ability to grow rapidly.