It seems possible that people questioned the price, not because they were confused about fractions, but because they thought that the burger was too low in quality to justify the same price as a McDonald's Quarter Pounder.
[edit: While they say that A&W's burger outperformed in blind taste tests, those taste tests could have had methodological differences (or even just a different sample of the population) that would make it impossible to compare their results to those of the focus groups.]
[edit 2: New Coke also outperformed regular Coke in blind taste tests and was also priced the same. But it was a massive failure, and not because of mathematical confusion.]