Starbucks has many customers, most of whom are not aware of the Delta SkyMiles partnership.
So I think this promo was a very small part of "the stars dilution". Starbucks probably had this doubling of the coffee reward in mind well before the Delta partnership.
QUOTE=dx3003;34872860]part of the dilution came from the ease of getting stars as a SkyMiles member[/QUOTE]