A while back, I was trying to explain to my daughter why illegally copying music was stealing. To my surprise, she simply could not see why it was considered stealing. Since she's an intelligent, fairly sophisticated, and reasonably honest person, her response made me question my assumptions.
I've since decided she's right. Copying music may be somewhat like trespassing, but it's almost nothing like stealing.
The intangible property of value that you own when you create music is the copyright, not the song itself. The copyright can be bequeathed in your will, sold, etc., much like tangible property. On the other hand, you don't directly have ownership in any of the copies of the song.
When I copy a copy of a copy of a copy of a copy of a song, I'm not stealing or directly affecting your ownership of the copyright in any way. This is very similar to when I walk across your land uninvited, I'm not stealing your land or directly affecting your ownership of the land in any way. I merely trespassed, which while illegal in the US (but not other countries such as Sweden), is a much different level of moral and legal breach than theft.
Indeed, unlike laws against stealing which are derived from fundamental western morals ("thou shall not steal"), copyright is only a legal issue, has no objective or traditional moral component, should only exist only to further the arts and sciences (according to the U.S. Constitution), and is really only even relevant if a substantial majority of people believe it has a net benefit to society (which, to the dismay of the record companies is becoming less and less true by the day).
I think that calling copyright violations stealing is derived from the legal marketing of the recording industry and other stakeholders in monopoly rents from copyright. After all, what better way to get the public on your side than to try to assign an erroneous but much morally stronger word to the violation of a law you strongly care about.
My daughter and her friends are right, copying is somewhat like trespassing, but not at all like theft.