@ Double fail on you
You are comparing apples and oranges.
If you walk in HMV and take a $20 DVD, HMV is out "REAL" money. The money they paid for the DVD (the studio don't really care as they got their money), something like $12, so they will have to sell two more discs to make up that loss. There is also $8 in "maybe" money that they might have got if they were able to sell the DVD, but it's only maybe as they might have had to return it on stock balance or bargain bin it if no one bought it.
If you download the same movie from some torrent site there is no "REAL" money. No one has less money then they had before you downloaded the movie (unlike HMV in the example above). There is only "maybe" money. If you couldn't download it maybe you would buy a copy, or rent it, or watch it on pay TV... But maybe you would not watch it at all, or as others have said maybe you like it so much after downloading it that you will buy a copy where you would not have before seeing it.
HMV know the number of items that walk, and they know number they sell, and what they pay for them in real money. They CAN calculate how much they need to charge, and how much to spend on cameras and sensor tags and such.
You can't calculate maybe money*.
How much extra are you going to charge because maybe someone didn't buy it? Who is going to charge extra? The studio? HMV? Both?
If you charge more will the people who actually buy keep buying the same number of items (pay you more in total) or just spend the same and buy fewer items? Or buy less, or nothing and spend their money on beer? (the amount of money people have is fixed)
*The studios guess. They pull a number out of their ass (a big one) then multiply it by another number they pull out of their ass because the first number was probably too low, then multiply it by the SRP and say they lost $6 bizzillion and it's a loss of 214,341 jobs and funds terrorists, and other bad stuff...