Perhaps the conclusions drawn by the experiment (if they are correctly paraphrased in the post; I didn't read the full paper yet: http://www.d.umn.edu/~dglisczi/4501web/4501Readings/Shiv(199...) are valid but I think it would be pertinent to consider that perhaps rather than being 'cognitively taxed', those 7-number participants simply felt that they worked harder and therefore deserved a better prize. I often find myself making similar justifications if I've pushed myself hard in a workout or followed my diet faithfully.

It would be interesting to see an experiment that 'cognitively taxes' participants by having them perform a task that is not considered positive. Memorizing a number elicits a feeling of accomplishment that may contribute to the justification I described above.