Zero is not just another discount. Zero is a different place. The difference between two cents and one cent is small. But the difference between one cent and zero is huge!
They reacted to the explicitly priced gift in exactly the way they reacted to cash, and the gift no longer invoked social norms—by the mention of its cost, the gift had passed into the realm of market norms.
People are willing to work free, and they are willing to work for a reasonable wage; but offer them just a small payment and they will walk away.
In fact, after thinking about money these participants were less willing to help an experimenter enter data, less likely to assist another participant who seemed confused, and less likely to help a “stranger” (an experimenter in disguise) who “accidentally” spilled a box of pencils.