Just like WoW accounts, Diablo III accounts are worth real money. Blizzard has had experience dealing with compromised accounts for years. This is why it introduced the Battle.net Authenticator, a second level of security that makes it very, very difficult to get your account compromised. Authenticators don't make it impossible to get your account compromised, but they do make compromising your account much more trouble than it's worth in the face of mass keylogging, which is how accounts are normally stolen.
Some people who haven't had a WoW account before but bought Diablo III were undoubtedly surprised when their accounts were compromised, which is understandable. An editor at Eurogamer had his account hacked and responded with an article suggesting that players were getting their sessions hijacked by joining public games and that people were getting compromised with this method even with authenticators attached to their account. Unfortunately, sites all over the internet picked up the story and also reported the session hijacks and bypassed authenticators as fact.
The problem is that neither of those things were correct. In fact, Blizzard says it's actually impossible to do with Diablo III due to the way the infrastructure is set up.