Most likely they only had a limited quantity of the title that they were selling at the sale price, so when that allotment was sold out they marked the item itself as sold out. Amazon does this a lot too, especially with their Lightning Deals.
I thought normal operating procedure would be if you are going to place an item on sale, actually sell out of said item and then mark it as 'sold out.' Not sell however many of said item and then once your system has sold 90% or whatever of said item, have your system state it's been sold out when in fact it isn't.
It's just a bad practice no matter how one tries to spin it. There is no way it is coincidental that their system indicates that they are sold out and the instant they end the sale price of it they all of a sudden have stock of it once again.
Do like I did and e-mail Customer Service and Corporate Headquarters (both of which are a headache to obtain e-mail addresses for on-line [I wonder why]) and communicate with them that you find this a deceiving and unscrupulous business practice.
You know how it works these days. Unless enough people bring it to their attention and complain about such practice, they will continue to use it.