Piracy is not theft. It is infringement. There are two different words in the laws used to describe these because they are two different things. Here's the simple (and subtly incomplete) way to distinguish them.

When theft has happened, the property is somehow unavailable to the owner. If I have a pen and someone steals it, I cannot pick up my pen and write. If I have a hundred dollars and someone steals it, I can't take my wife out to that special dinner I had planned. And so on. You'll note (as with the hundred dollars) that the property in question doesn't have to even be physical. That hundred dollars could just be a number in my bank account that's now 100 lower. It remains theft because that property is now no longer available to me, the rightful owner.

Infringement, conversely, does not remove the ability to use the property from the owner. If you produce a song and I make a copy of it, you still have the song. You can still play it. You can still try to sell it. You can do whatever you like with it. I have not removed it from you. If you have an idea for a product and I use industrial espionage to get that idea for my own use, you can still use it. It has not been removed from you.

Now what has potentially been removed from you in infringement, which is why there is both civil and criminal infringement, is the possibility of profiting from the work you put into the property. If I take your song and make it freely available, you can't sell it. You haven't lost the song, but you've lost the ability to pay rent or eat for sales of it. If I take your product idea and sell it cheaper (because I didn't pay the R&D costs for it, say), you haven't lost the idea, but you've lost the ability to recoup the expenses it took to get the idea into a usable state. So I'm not saying that piracy is always good, nor am I saying it's always bad. I'm saying it isn't theft.

"But if you get it without paying $10 (to pick a number out of thin air) for that album, you've stolen that $10 from me!" No. No I haven't. Unless you want to also make the risible claim that if I choose not to have the album at all I've also stolen that $10 from you. You never had the $10. Ever. It was never your property. Ergo it cannot be theft if I merely don't give it to you, no matter what the circumstances leading to that decision not to give it to you. I've infringed, sure. But to call it theft is just plain wrong. And I don't mean "wrong" in a moral sense. I mean "wrong" in the very technical literal sense.