Most online games where this issue could arrise have lines such as the following in their TOS:

"Notwithstanding anything to the contrary herein, you acknowledge and agree that you shall have no ownership or other property interest in the Account, and you further acknowledge and agree that all rights in and to the Account are and shall forever be owned by and inure to the benefit of Blizzard."

And other lines that indicate any item/character/etc that you "own" in game is in fact property of Blizzard (in the case of WoW)

If you take something from another player in WoW/other game with said TOS clause, the only rules you can be breaking are Blizzard's own, you can't be stealing unless you find a way to remove it from their servers into the real world (unlikely...) and in that case you're stealing from the game company, not the player.

So next time you're accused of stealing something, remind them that the original owner still owns the item/s in question and there has been no theft :)