I was actually thinking the complete opposite. Since they found nothing of value in the wallets, they were easy to part with and send back. If they found $300 in cash, the finder may have felt more inclined to keep the wallet or at least the money. A good follow up test would be to see how often the money comes back with the wallet.
I was not trying to speculate what people would do if they found a wallet full of money, I was just trying to interpret why they sent back a wallet with a baby picture but did not send back a wallet with nothing of value inside.
