Hansel and Gretel Marketing - How Barneys Entices Customers to Come Back (picwing.com)

"The tactic works pyschologically to make her feel indebted"

I've had similar feelings toward free offers and I'm sure it's a common occurrence.

This and other sales tricks are explained in Cialdini's "Influence: The Psychology of Persuasion".

I really like that book because it made me feel more conscious about those tricks, even though it's mostly stuff "everybody knows".