It only takes a little "nice"....

I've just arrived in Tampa, ready to present my workshop on communication and motivation to Florida administrators.  I got here early and decided to go shopping.  (And why not?)  While getting ready to check out at a department store, a lovely lady with a beautiful French accent asked me if I would like to open an account.  Knowing that we get more perks with my current credit card, I simply said, "I appreciate you asking but no thank you."  She turned around and said to me, "That was so nice!"  Almost ready to say, "You know I just said 'no', don't you?"  she clarified, "Most of the time, when people are not interested, they are not nearly as nice as you are."  Hmmmm....I thought.  Why not?  What harm does it do to be nice even if we are saying "no" to an offered service?