Because that is just not how things are done, if I walk into a store and go to pick up an item that I know should be around $20.00 but hey there is a price sticker on that item listing it as $6.99, I'm walking out of that store with the item for $6.99, thats just the way it is, if your employee screwed up with the price gun thats your problem not the customers. Hell that has actually happened to me and I wasn't even trying to get the item at the lower price, I was just trying to let the guy know it was priced wrong, he simply led me to the self checkout rang it up and changed the price.
They advertised a system working one way, then when we finally see it, it's another way.