Diablo 3 isn't a single player game. Even in interviews, Blizzard stated that it is not a single player game. It's the people who wants to play it like a single player game is voicing complaints when they knew full well Diablo 3 is an always online game. If they didn't want it to be always online, they shouldn't have bought it.
It's like buying Guild Wars 2 and complaining they can't play it off-line. Or World of Warcraft. Or any other game that had already explicitly stated it requires an internet connection to play.
To be fair, it is a game that is/was, traditionally, a single-player game. DII was the same...with multiplayer options.
This type of game caters to a very different crowd. Many of them probably aren't interested--AT ALL-in WoW or any type of online game. They just want to mindlessly hack at things, level, and pick up shiny objects. That's it--that is all they ask.
The gaming world has been unkind to these people for many years, and this was the one golden moment for them, when Blizz was expected to finally return to their roots, and give them what they were clamoring for. Despite what was said about the game before it was available, I think it is reasonable to accept that these fans expected to still be able to play this game the way they wanted.
They are unaccustomed to server downtime issues--again, they don't care about online gaming, so this is not something they expect. I imagine "always online" was simply interpreted as "security necessity," and they were able to shrug it off as a simple, "fine, as long as I can hack and slash to my heart's content, no reason to bitch about annoying DRM practices and such."
Once one realizes that this game is now inextricably tied to another game, an online-only game, whose whims and foibles determine, absurdly, the ability to play this other game, one should be able to understand this type of frustration from a crowd that has no desire to play this other game, due in large part to the annoyances inherent in playing such a game, annoyances which they rightfully (whether it be through ignorance or not) assumed would be avoided.
To demand that a crowd that dislikes online games and MMO playstyle adapt to the "new world order" of MMO frustrations (server mechanics) in a game that is not expected to be an MMO is being a bit shortsighted.