It's a good question. There are probably two elements to how this might break down in practice.
One the one hand, you could think about the risk of an enforcement action. However, even before you get to the risk of enforcement action, I think there's also the potential for a chilling effect, where businesses hear about the rule set, maybe understand to some degree or partially the rule set that's being imposed and feel it's too burdensome to take on the risk of advertising a sale that could be challenged. It might in some ways be read or misinterpreted as a restriction against offering sales or discounts.
By making the process of offering a discount potentially more complex or contestable or challengeable, it could, in fact, make it harder for a small retailer to put on a sale. That extra burden might mean, in fact, they don't go ahead and offer the discount.