Amazon's Top MP3 Sales was from a Freebie

So it turns out that Amazon's best-selling MP3 was actually from Nine Inch Nails's album that they pretty much gave away for free. A few questions come to mind:

• Why was Amazon selling it? I guess the answer there is that people were buying it.
• Why were people buying it? Hopefully, they were supporting Nine Inch Nails and Trent Reznor.
  
• Is giving things away a good business model? It seems like a smart move by the band, and it goes along with the post I made a while back about Neil Gaiman releasing one of his books for free. Definitely seems to be a good way to get things out there (and make money too). People will try something for free that they would never pay for, and some of those will like it enough to pay for it next time (does that almost sound like how drug dealers work? "The first hit is free, but you pay after that.")