And guess what, a game that has multiplayer elements != online game. There's this thing that's been around for decades called LAN. There's also split-screen and same-screen multiplayer games run from a single client. Multiplayer games can be offline too, so your arguments are the ones that are flawed.
The truth is, there is no good reason for Diablo 3 to be an online only game, and I doubt even Blizzard themselves, if they were being 100% honest, could answer the question of why.
The game could easily have two or more components that are 100% separate from each other. There can be a client only offline mode for single player that would have no effect on the online only Blizzard host authoritative economy. Therefore, the integrity of the RMAH can't be the definitive answer. Maybe they just wanted to "force" more traffic into the online zones because a larger population = a higher chance that the RMAH will be used.
Piracy is also a popular scapegoat, but the game will definitely be cracked at some point. It may be a half-baked, year-late experience, but it's still gonna happen. Now maybe I'm a bleeding heart hippie philanthropist, but if I'm in business, and I've made several BILLIONS in past ventures, I'm sure as hell not gonna screw a large portion of my legitimate consumerbase to delay the actions of a small percentage of non-paying customers. If I'm an auto dealership, I know I run the risk of a few cars being stolen off my lot. That doesn't mean I go out and require every person that's purchased a vehicle from me to scan their driver's license and handshake my customer database before they can start their car.
Diablo 2 already had a great system, there was single player offline, "open battle.net" for taking single player characters online, which was riddled with cheaters, but guess what, you didn't have to join public games with those people, and "closed battle.net" which had separate ladder and non ladder realms. I'll be honest, I was on closed battle.net ladder 90% of the time I was playing D2. That means I treated it mostly as an online only game, because there was an incentive for me to do so in the form of exclusive powerful items. But the option to play single player was there whenever I wanted it. The offline experience was better because it was lag free, and you could do things like spam meteors or run a skelemancer without any complaints. Hell, nine out of ten deaths online were due to lag, the only reason I never bothered with a hardcore character. And really, I could have used an editor to replicate the online only items in single player, but that doesn't quite have the appeal of building the runeword yourself. But when they finally shut down the D2 servers that's a fallback option we all have, to be able to play a game we paid for at any point in time.