You've solidified my point with your comment "if the advertisement is displaying a price that is considered to be a misleading attempt to entice people"... If you stand in a queue for however long on the pretence that you're buying a product for the price that is displayed, then they turn around...
Because the law says so. If you see something advertised at a specific price, the seller has to respect that price. The Trade Descriptions Act 1968 and the precedent set down by a case involving Tesco (I think) are where it comes from. Admittedly the law may have changed since I last looked at...