If you have a hard copy book and someone steals it, you're not only losing out on the potential sale price of the book, but the tangible value you have already paid to produce that copy.

Say the book is $12, you get $2, the publisher gets $5 - the book store buys it for $7, and sells for $12 making $5 profit. If you steal from the book store, they've lost a potential profit of $5, but more importantly they've actually lost the $7 they already paid for it. This is what theft is about, the value of a possession taken away, not the potential value.

With a digital book, each individual copy costs nothing. It costs something to make the original, but making a copy is free. Thus the only thing you've lost is the potential profit, which arguably you wouldn't get anyway as the person didn't want to buy from you to begin with - just because they downloaded it for free does not mean they would have paid full price if a free download wasn't an option.

With theft, you have a tangible loss. With digital piracy, the only loss is opportunity to profit.