There is no technically when theft is involved. Theft is theft. If you don't understand this, then you have some serious ethic foundation issues.
Meh, people can have different ethical foundations. Personally, if apple mistakenly sends me an extra charger, I'm not sending it back unless they send me a return label and arrange a pickup.
I know when I sent my MBA for repair a while ago, I was instructed to give only the machine. Now it didn't come back with any extras... But if it did, I dont think I'd be obligated to return them.