They didn't say that you couldn't play single player. Being connected to the internet is not going to effect you.
Fattypants, there will never be a day were you will be unable to play diablo3 because its too old of a game. They can always patch it later. Most companies aren't as worried about piracy after the first few years.
It's not piracy they're worried about. Piracy claims are all PR bullshit. They care about the used market and about controlling every thing we do with the game.