I've gotten those some of those emails because of YouTube's Video Identification. I'm guessing because they didn't automatically get pulled off that the record label doesn't care if you share it cause it's just spreading the word.

But I got this today from a music video "This video is no longer available due to a copyright claim by Warner Music Group" what are the chances of them actually doing anything other than just taking the video down? I'm thinking about re-uploading that same music vid but a different file cause it's quite popular. I'm doing the uploading through a host's server so what would be the chances of them finding me? :-p