If I buy a candy bar at the store for $0.99 and then you steal it, now you have it and I don't. I'm out $0.99 of value and you stole $0.99 of value.
But what if you steal the candy bar directly from the store? You got $0.99 of value, but the store didn't pay $0.99 for that candy bar; they may have only paid $0.40. What you actually stole from them was two things: $0.40 of inventory and $0.59 of foregone revenue.
Even if you drop $0.40 on the counter as you walk out the door, it would still be considered a theft because the store gets to decide what their retail prices are, not you. You're still stealing $0.59 of foregone revenue.
IP products essentially have an "inventory" cost near zero. Yet, if you take a song or a trade secret without paying for it, you're still taking their foregone revenue. Just like dropping $0.40 on the counter for a $0.99 candy bar, you're dropping $0.00 on the counter for a $0.99 song or $1 million trade secret. Reimbursing someone's inventory cost (even if it's $0.00) does not mean there is no theft occurring.
While the IP itself is non-rival, the revenue from each sale is rival; that is, if you acquire my IP via a pirated copy then I still have my IP, but I don't have the revenue from selling you my IP.