The options aren't just returning it or stealing it, they can simply leave it where it is to avoid the hassle of having to return it. Hence why having a custom of paying a reward might be beneficial for wallet losers in general.
The bug bounty isn't only about the money. It's also the company's way of advertising 'we aren't crazy assholes like those outfits you heard about on the news'.
(Yes, fixing the law would be a good idea. But in the meantime, a bug bounty is the solution.)