I think Notch intentionally did that. Say someone wanted to try the game, but not pay, they'd go grab the download and just play offline in single player. That person finds the game really enjoyable and tells his buddies about it. They get the game the same way, and enjoy it as well. They all then want to play and build stuff together, and so they all decide it's worth it to buy the game. So instead of Notch just getting one sale, he gets the original person and his buddies that he got hooked on the game.