United did used to serve Starbucks. Rumor is (or at least what was included in one of my b-school marketing cases) that Starbucks pulled the plug becuase either the United FA's or the galley equip couldn't brew the coffee to Starbucks exacting standards.

As the previous poster mentioned, this could be an issue--the restaurant or food's brand image could potentially be tarnished when 130 passengers all receive a half-cooked Boston Market turkey dinner or such. (Okay, perhaps Boston Market's brand wouldn't suffer much...do people still eat there? )