No, they can accept back up offers in case the primary offer falls apart. Some sellers want the reassurance of a back up offer.
This practice of bait and switch is just wrong. Just like a Car Salesman advertising a car that is no longer for sale. And then Realtor's wonder why they have a bad reputation. Am I correct that if the property is under contract the selling agent can no longer accept any offers?
I am sure as a buyer this could be a frustrating event and when this does occur we as agent are often somewhat unsure how to deal with it but being on site and honest with prospective buyers is the best policy I have found.