The auction house was never the reason for always online, says Blizzard.
Yes, the Diablo III auction house is no more, but does that mean the game will drop its always online policy? No, Blizzard tells Rock Paper Shotgun, dashing the hopes of those eager for a single player offline experience. In fact, the auction house wasn't the reason why Diablo III had to be always online; there were other compelling arguments, like Real ID and BattleTag social features.
The full statement reads:
We do not have plans to implement an offline mode. While the always-online requirement made the auction house possible, the auction house was never the driving factor in our decision to make the PC version of Diablo III require an Internet connection. The game was built from the ground up to take full advantage of Battle.net, which provides a number of important benefits, including persistent server-side character saves, a seamless PC multiplayer experience, cheat prevention, and Real ID and BattleTag social features.
So there you have it: the seamless PC multiplayer experience, cheat prevention and other fascinating bits and bobs is why always online is your lot in life, PC gamers. Never mind; at least the console crowd can play without a constant internet connection.
Source: Rock Paper Shotgun