There's a difference between downloading music (albeit illegally) and taking someone else's work and trying to pass it off as your own. When I download a Duran Duran album, I don't post it on my website and claim that I made the album and attempt to sell it as my own (as much as I wish I was Simon LeBon).
This isn't an issue of piracy -- it's an issue of plagiarism, which is quite different in my opinion.
How about we compare it to someone who put up a youtube video of themselves lip-syncing to Duran Duran? They're not necessarily claiming that the music is theirs, they're just using some random song they found to provide a background/prompt for their own performance.
Maybe its more fundamental? - People do it if they know they aren't going to be exposed (like downloading illegally) and they won't if there was a way to expose them (say via facebook/blog)? Just a theory.
I still believe downloading illegally is wrong, but thats my opinion.
This isn't an issue of piracy -- it's an issue of plagiarism, which is quite different in my opinion.