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).
