Would it still be theft if the money in your wallet was money that you found on the street the day before?
(Also even when a wily Sicilian complains that "You're trying to kidnap what I've rightfully stolen".)
All they said is that the other person, Wallet-Dropper, is something less than the prima facie legal owner. This permits several possibilities, but in every case Wallet-Taker still seems to be doing something wrong:
1. If Wallet-Dropper is a thief, then Wallet-Taker is just a thief stealing from a thief.
2. If Wallet-Dropper is the well-meaning temporary custodian of unclaimed property that they cannot (yet) claim as their own, then Wallet-Taker is a thief, taking to deprive the legal owner (wherever they are) of their property.
3. If Wallet-Dropper is the new legal owner of the discovered cash, that's theft by Wallet-Taker no matter how much you think Wallet-Dropper "didn't really earn it."