Keep it if you want to. It is your morales that suffer. Be the better person...
What, from not giving back a few bucks to a company who has billions of dollars in the bank & whom profits greatly from your business? Personally if Apple didn't miss a charger that I didn't send back I would keep it and not lose any sleep over it. If it belonged to an individual person or a non-profit organization then it would be a different story - But it doesn't. It belongs to a rich company that makes handsome profits off you and me.