The replies they sent to emails said "it's OK; please keep or donate the money as you please".
They did in some cases (seven cities in the Czech Republic and Switzerland) then drop by in person to try to retrieve the wallets, but (unless I misunderstood) that's only about 1% of the wallets.
Also, the wallets have a cost too, so it is not only the equivalent of $13 being lost in some cases.