This is kind of silly. Doing street level market research is not a new idea and approaching people directly isn't some huge revelation. There aren't so many entrepreneurs in the world testing ideas that you would ever be affected by this more than once.
Don't be afraid to approach people and ask for their help! Most people would love to know that someone cares about their opinion. There are huge online communities that run entirely off this principle, like Yelp or Quora. If someone's not interested, they'll tell you.
This is fine on the street, but not inside a Starbucks. As soon as you start approaching customers inside a business, they have the right (and obligation) to ask you to leave.
The OP had a table set up with the gift cards, and was only (I think) engaging people who came up to the table. Presumably he had the permission of the shop management to set up on the table. I'm happy with that provided he wasn't hogging the big table at peak times. If you are not interested, just don't go to the table.
I'd hate people walking up to me at random inside the coffee bar (UK reserve perhaps).
Don't be afraid to approach people and ask for their help! Most people would love to know that someone cares about their opinion. There are huge online communities that run entirely off this principle, like Yelp or Quora. If someone's not interested, they'll tell you.