It is likely that someone in the recording studio made a CD copy of Metallica's work in progress and converted it to MP3. Metallica needs to look closer to home and find out more about the people they associate with and whether they can be trusted. At least, it was just a song. Ask Pamela Anderson and Tommy Lee about trusting people that work for you when you're famous. You can tell the difference on any decent audio system.
Most people still buy what they like even after downloading it at no cost.
Many studies have indicated this to be true. This is the big issue facing the record industry. First of all, even if someone pays to download an MP3, it can immediately become available for free. Second, an MP3 is not tangible. If your computer crashes and you need to format your hard drive, your MP3's are history.