I got the point of it, I just think it's horrible execution.
If you have a business idea, and you cannot find a way to solicit honest feedback from an ad-hoc consumer panel, then you should probably check yourself.
I don't understand why you need the whole deceptive angle. Go to Starbucks and approach someone and say "hey... I've got this interesting idea for [a way to do X better/faster/cheaper]. I'll give you a $5 gift card if you don't mind spending 15 minutes giving me your honest feedback."
The other fail of the idea was the whole trying to maximize $20 at Starbucks concept. Many people are going there to get a drink that will likely average $3 or more. Offering to buy someone a $1.50 black coffee when they came in wanting a $4.50 latte isn't going to get you off to the right start. Even spending $100 for 20 $5 gift cards would be cheap feedback, and (IMO) a more realistic approach.