I mean why does anyone buy anything at all in life? I mean we could just take everything at gun point - booze, televisions, games, food, you name it. It's all public domain.
Wait, did you just compare piracy to taking things at gunpoint..? Really?
Anyway, as usual, you're missing the point of the question. cliffski assumes that people won't buy anything they already have. In this case, have != legally possess and if that's the case, everybody "has" pirated stuff. Movies, music, games, TV shows, etc, are all readily and easily obtainable for free from the interwebs. Is it illegal? Sure, but laws only hold value when they are enforced. Piracy laws are about as enforceable as jaywalking laws at the moment so they are pretty meaningless. So again, why does anyone buy movies, music, games or TV shows when are they easily obtainable for free? In essence, we already have all these things. They are just a few clicks away. So why does anyone buy anything?
Do people buy these things out of moral obligation? If that's the case, why would it be any different with pirates? Why would a pirate be any less likely to buy a game they enjoy? After all, isn't that the reason people buy games after playing demos? What you and so many others fail to recognize is that there is no absolute definition of morality. Morals vary from person to person. Take murder, for example. The murder of insects is generally accepted and even encouraged. The murder of furry animals is generally discouraged. The murder of humans is generally forbidden unless it's to protect yourself or someone else. Context is everything. The act of piracy is completely meaningless. It's what you do after the act that matters. If you enjoy a pirated game and buy it, the developers are rewarded for their work and you made an informed purchase. If you dislike a pirated game and delete it, the developers are not rewarded for crappy work and you save yourself from making an uninformed purchase.