For a more intuitive explanation, imagine a transparent, padlocked box. This box has holes so it's easy to put cash in, but you need a key to get cash out of the box. Each user's cash is stored in several boxes, and only the user that has the key to open the box can take out the cash. Who owns the cash inside these boxes in public places? Since only the person with the key can open the box and use the cash, the person who holds the key is the owner.