You're right. But in my anecdotal experience, there are about half as many adding the surcharge as providing the discount.

Ultimately, my point is (not even addressing the legality of Visa's compulsory pricing attempts) the cost of upfront analysis to set pricing appropriately is easily dwarfed by the cost of labor and throughput when adjusting the price by employees at the point of sale. Maybe the merchants adjusting at sale are getting a benefit in the form of customer awareness, but I have doubts.