It's because despite what economists believe, prices are not determined fairly by the market but are dictated supply-side. So this kind of "testing" is one of the more blatant ways in which consumer trust is violated in the name of maximizing profits. It makes it obvious that you're not paying a fair price for a service or product, but a price that was dictated for someone else's convenience without taking your interests into account at all, violating the social contract.