We all have an intrinsic motivation to do the right thing, e.g. to return something that does not belong to us. A reward adds external incentive to the internal motivation to ensures the right thing is really done --> reward substitutes moral.
Personally I think the 25% are ridiculous high. But then, it explains why people were so surprised when I found in RU their wallets/cells and returned it.
Anybody knows, how much rewards are usual in other countries?