When you take a bottle of water, the cost of inventory (which captures the entirety of the amortized cost of production) is less than half of the harm to the shop keeper, because the markup on bottled water is well over 50%. The primary harm is the revenue the shop keeper will forego because he cannot sell that bottle.
In the digital realm, using a copyrighted piece of content without permission causes the copyright owner--who did have to invest in the original creation--to forego revenue as well.