It's about price discrimination. The first thing is finding out exactly how bad their customer wants the movie, so that's why there's availability windows, to see how a given customer's behavior changes based on availability of the product and substitutes. With that they can then maximize profit and charge $15, or $14, or $16, or whatever the exact maximum you're willing to pay for it is. And that's also why it's frustrating: your frustration is literally what they are trying to gage and measure and maximize.