Well I know for sure in Ohio, there's something called the Ohio Consumer Sales Practices Act. It CAN, in certain cases, violate that act if they don't honor the lower price because that's the price that the consumer expects to pay, if it was advertised at the cheaper price.
It's like selling something on ebay..if you have something you mis-priced and put it on a buy it now, then have someone buy it before you fix the price, you're still obligated to sell it to the person for the original price. My mom had a friend once who had that happen to her and the buyer won their case.
A lot of stores do just honor the accidental lower price because in certain courts, that CAN be seen as bait and switch or deceptive advertising, whether it was accidental or not. And along those same lines, while it's hard to prove that someone deliberately mislead people, it's just as hard to prove that they did NOT deliberately mislead people.