Comment by superfrank

They stole from the amount they would have legally paid to buy a copy from the copyright holder.

Think about it like sneaking into a movie theater and watch a movie without paying. The theater was going to play the movie anyway and, assuming it wasn't a packed theatre, I didn't deprive anyone else of their ability to watch. It's still theft because I'm getting something that costs money for free and depriving the theater of the money that they're owed.