agree with both of you, they could have easily done that, but they didn't and the game as it is is not singleplayer.
The online requirement doesn't affect the nature of gameplay. The singleplayer component is no different in D3 than it is in D2. Forcing you online to play singleplayer doesn't take away the fact you are playing singleplayer. That'd be like saying that Assassins Creed had no singleplayer.
Heck, let's just say for the sake of argument that it is a MP game. People would have a right to complain about server queues on launch day and seek compensation for downtime and other problems with the service like they do in other primarily MP games. The consoles even do that stuff for their online services. So saying "people need to think of it as a MP game" isn't really much of a defense either way, Blizzard is still accountable for ensuring that people have a playable experience to the best of their ability.