For older Oyster cards that have a deposit, you can still reclaim that £5.
The fee/deposit payment is part of it (prevention, it not seeming a disposable item) but with a deposit (there's no good way to say this..) there's also the incentive for the homeless or otherwise desperate to clear them up, since there's some guaranteed value. (Which could be a bit less than £5 due to negative get-me-home-at-night balance, but I don't think it's allowed to go as far as -£5, so a card with a deposit on it is always an asset.)