Its very possible to have a computer, and not the internet. I have a couple of friends that just want nothing to do with it, but have a computer because they play games on it.
I know. It is very shocking. Diablo started as a single player game. There is no reason why they couldnt allow an offline option for people that do not want a part in the online community.
I purchased Starcraft 2, not because of the multiplayer, but because I enjoy RTS games against a computer. Starcraft has an offline option. You get zero achievements. But you can at least play the game, and enjoy the story.