One of the core problems with starbucks is the over roasting of all their beans, this is done to "ensure" consistency between stores. Mix that with enough marketing and a general populous that cares more about being hip and don't know any better, consistency becomes a major factor for them. If one store produced a better quality/tasting drink then you could have fragmentation of the brand name. It's the unification of the brand name that sets them apart from the mom & pop coffee shops, so consistency becomes one of the key factors to ensure that setting apart. Now to do that at a cheap cost the easiest way is to over roast the beans so that it doesn't matter where they are getting them from or the original quality..
While i don't agree with it, and i don't care for their coffee, i can fully understand their strategic decision to do it, and it was and is proving to be a very wise decision.