I guess technically you are purchasing a licence to watch the film, within the constraints set out in the iTunes Terms of Service, which permit Apple to withdraw your access to the film at any time, for any reason.
I wonder how long until we'll have restaurants where you "purchase" a license to taste their food, but they may - at their sole discretion - withdraw your access before it reaches your table ^_^
Not to that extreme, but there are certainly restaurants that take your food away if you don't finish your meal within, say, half an hour.
That's pretty extreme. What's the point? They can't serve the food again to someone else.
The point is to free up the table so that they can seat another customer.