The relevant regulation (https://www.ecfr.gov/current/title-7/subtitle-B/chapter-II/s...) says "No retail food store may single out coupon users for special treatment in any way.". Imagine, for example, a retailer that offers SNAP discounts only on the worst cuts of meat they otherwise have trouble selling; you can imagine why a SNAP recipient might feel uncomfortable with that, even though in principle it's a Pareto improvement.