Companies are purchasing labor (a product). Trying to undermine the costs because they have inside knowledge on how the meat is made (or where the meat is made) is just them using their leverage to extract a better deal. Totally normal tactic, they do this in every facet of their business. Employees should also grab as much leverage as possible from shopping their product (labor) to competitors or banding together with others (union/guild/etc) to get the best deal.
tl;dr It makes rational sense for companies to want to pay different per location. It makes rational sense for employees to say "cool, but no." There are many examples of price not being location-dependent in our capitalist system, at least many I have experienced at a consumer level. But maybe you are onto something and online consumer stores should all start changing the base prices once they see your zip code/state/type of credit card (I'm positive there are already some out there that do things along these lines).