Blizzard's online technologies VP Robert Bridenbecker admitted to MTV Multiplayer that he was "actually kind of surprised in terms of there even being a question in today's age around online play and the requirement around that."
For one, Diablo II had addressed this issue. Furthermore, what does the single player experience have to do with online play. It also seems Blizzard doesn't like the idea of players using their own LANs for some same area gaming.
It also doesn't address the idea that broadband is not only not widespread, but some broadband services have bandwidth caps. It also doesn't address the fact if the Blizzard servers go down, everyone gets screwed, internet connection or no - just ask Electronic Arts when their Dragon Age servers went down, blocking people from using their paid DLC. Electronic Arts was also there first-hand for the outrage aimed at Spore, and how sales tanked in part due to the DRM.
It also doesn't stop piracy, and even if it did, Blizzard wouldn't see extra money because pirates don't pay for software, and won't purchase it even if they can't crack it. On the other hand, it will lose money from legit subscribers who don't have constant internet. (It doesn't help that Blizzard has dared pirates to crack the game - never a good idea.)