The pirated version any idiot with a bittorrent client downloads, for free, actually works. The one bought at Wal-Mart, for money, does not, or at least not until after a couple hours of insanity.
Guess which is going to be more popular?
Additionally, consider how many people here are honest enough to buy a game, but end up grabbing a no-cd crack or an entire pirated version of the same damn game, jut to get the freakin' thing to run? I'd bet they have gotten piled into that "80%" as well.