That's the very definition of what we're talking about here. We are duplicating something we want and keeping the copy without paying for it, thus the manufacturer doesn't get money for the item we obtained. It doesn't matter that we don't physically have a copy, because it's the data that matters.
Let's say you go into a store, tear open a copy of World Of Warcraft and take a picture of the CD-Key. You then go home and activate the key on your battle.net account. You didn't physically remove anything from the store, but now they can't sell that copy of the game because with the key used, the box and all its contents has no value whatsoever. This creates a revenue loss to the store and/or the company they got the game from. Since the game itself can be downloaded and installed without owning anything that comes in the box, that cd-key is the "item" you're buying..or stealing, in this example.