Piracy is theft, but what really hurts is the reactions companies have against piracy -- they make legitimate ownership more cumbersome!
Theft, by definition, transfers value from the original possessor use to the thief - a zero sum transaction where the thief gains at the owner's loss. Copyright infringement does not do so. It only deprives the original possessor of potential future value. (You could even argue within some legal jurisdictions that there is copyright infringement only when the pirate derives commercial gain from the copy.)
When I first heard about piracy, it meant people copying CDs and _selling_ them.