It could simply be Starbucks didn't want to be associated with the project, or they never gave Mobiquity permission to use their name as an endorsement. This stunt could have caused them to notice.
And this could still be the case while a marketing team at Starbucks has decided that this card is a great tool that they can sink a couple thousand dollars into slowly to get people in to Starbucks.
Like you say, it is pretty tempting to assume the worst because nobody wants to be the last person to know they've been duped.
I don't think it really matters in this case whether it was sincere or not, but I guess people don't like to feel like they've been played.