I think something might be a bit at odds here between the gameplay that larian has created and the multi-player possibility. They have been talking a lot about poisoning a friend and dueling motivations. In order to have this then yes, a random could steal an apple and grief your game.
That actually sounds like a very serious problem.
Larian's designed the game to have some amount of mischief. But it gets out of hand, as you say, if they can just steal an apple and turn the whole town against you. Another problem with the way NPCs overreact to stealing.
