That doesn't just apply to multiplayer, though. OP said Diablo 3, which is multi OR single player. Many games now use those phone-home anti-cheating measures (or drm measures) even when you aren't actually playing with anyone else.
Although they did, there were some item duplication bugs. And even that was irrelevant, since there were quickly popping up small armies of item / gold farmers; every once in a while they'd get banned, but by then they would've earned enough money to buy another fifty copies of the game and continue.