Hee! Jesus, that's bad. You're right though, loads of companies seem to be doing it. I'd imagine it's because a demo can potentially bludgeon a game's success though, since if people try it and dislike it, they won't bother getting the full game.
A common strategy seems to be allowing most people to go and buy it, and once everyone who's going to get it without trying it first has done so, they'll release the demo. Sneaky, but it works.