Why can’t we have the option of renting a movie (say from Apple) and then if we want to purchase the movie we just watched, make the cost of renting go toward the purchase price?

Example: I just rented “Blackberry” for $6.99. If I like it, and I move to purchase said movie within X hours (or whatever), the purchase price would move to $8, instead of $14.99 ($6.99 rental free + $8 = $14.99)?

I’m always confronted with the question, even before I watch the movie I’m renting, “Will I ever watch this twice?” Because if so, I might as well buy it.

So frustrating.