No one will investigate the theft of Diablo 3 gold or WoW gold. In fact, even Blizzard support will not do anything, not to mention the police. So yes, in most countries(which did not recognize Bitcoin as currency or property), stealing bitcoins isn't illegal...
Theft of any tangible property is illegal. Period.
'Splain me why Trendon Shavers owes the feds several million dollars?
I've not read Blizzard's TOS. Never had a reason to. I rather imagine that they state that 'the user acknowledges that any 'theft' of in-game currency is not a theft of any property of the user'. Hell, the TOS may even state that all in-game assets are the property of Blizzard.