It's called a lost leader in retail sales. Meant to hook you in to coming, with the hope you'll buy something else, and hopefully at the full retail price.
Many years ago K-Mart tried that on me, right before Christmas with a toy all the kids wanted, on sale, on such a date. I was there when the doors opened, third person in the store, headed right to the toy department, none in sight. I raised heck with a floor person, and not quietly I may add, who got the manger, I then started in on him. Two minutes later he came out onto the floor looking for me, seems he had found one in the back room.