Retailers don't "pass on the savings". One retailer will lower the price if they don't have to pay as high a Visa fee - to attract more customers. Other retailers will then follow suit.

Free shipping wasn't much of a thing until one retailer decided to make it one :-)

Or they will increase prices later than they would have. Which I think is most realistic scenario that will happen if cap is set. Prices don't go down, but they will go up slightly less over next few years.