They controlled for this (in the version of the experiment for which the results are reported in the article): they offered it as part of a campus cafeteria mea

They controlled for this (in the version of the experiment for which the results are reported in the article): they offered it as part of a campus cafeteria meal (and integrated it into the cafeteria billing), so the subjects were already paying for something whether they bought a chocolate or not. Source: http://people.duke.edu/~dandan/webfiles/PapersPI/Zero%20as%20a%20Special%20Price.pdf http://people.duke.edu/~dandan/webfiles/PapersPI/Zero%20as%2... (see Experiment 3)