Based on this article it sounds like they pay enough to want a law to make it less:

https://www.cnn.com/2022/09/20/business/credit-card-fees-vis...

Costco gets away with 0% because they offer exclusivity and because they cover some of the cost of the rewards programs. I guess Walmart et al did the math and figured they would lose too much business.

The average Costco shopper makes over $100K a year, so getting another card just to shop there isn't a problem for most. Can't say the same for Walmart.