That's where I think a lot of the confusion on both sides of the issue come from. Generally theft is understood to be taking something not belonging to you, thereby depriving the rightful owner of it. But in the age of "intellectual property" that's been expanded, and not always in very sensible ways. With the recordings in question, the "theft" is actually the creation of a duplicate copy. The thing being allegedly "stolen" is the potential customer, or more precisely, the money he might have spent purchasing the song through some legal medium. That's a lot more difficult to popularize as theft, and it's one of the reasons moral outrage against thievery isn't making a dent in song sharing.
The urge to protect intellectual property is understandable - it encourages the producers by providing opportunities for profit. But there are powerful free-market reasons to discourage intellectual property also - after all, where would American industry be if Henry Ford was the only one allowed to use an assembly line?
Obviously, our current laws haven't attempted to drive home some sort of pure philisophical point on the issue. They have attempted to establish a reasonable and enforcable balance. But that sort of balancing act has to adapt to fit the situation.
With the current situation of the recording industry, it is simply no longer reasonable to try to prevent song sharing. The technology to record and distribute, once expensive and specialized, has become extremely cheap and nearly universal. Just as it would be ridiculous to prevent someone from hearing a tune and singing it later in the shower, it's growing ridiculous to try to stop someone who has a digital copy from making another and sharing it. While the RIAA might benefit from an intense and expensive governmental enforcement initiative, society overall would be more hurt than helped. It's time to redraw the line in a more reasonable manner.