I think it's twofold. You've got it right that the plagiarism is probably the biggest reason, but the other reason is that they're using this "stolen" material for their own personal gain. If I download some MP3s and listen to them myself, there's no personal gain (besides the literal gain of having the MP3, of course). If I download some MP3s and put them in my product, now I'm illegally benefitting from someone else's work. I would be willing to call that stealing the MP3s even if I'm not trying to pass the MP3s off as my own creation.