Answer:
The "tin" you insert into a safe deposit box receptacle belongs to the bank. If you would not let the customer borrow an office chair, stapler, or adding machine for a few days, then you would not allow them to remove the tin from your premises.
That's not a function of law or regulation, just the way you look at bank property. Tins can be bent and rendered unusable fairly easily and, if they are never returned, can render a box unrentable.
First published on BankersOnline.com 9/13/10