I always love this thinking, "I didn't take the chance to steal from you when I had it, I deserve a discount!"
The reasoning is "Giving incentive to act good instead of only punishing for doing wrong".
If you find a wallet on the ground, do you bring it to the police? I know I've done so several times. Mostly I don't hear anything about it afterward. Once I got a thank-you card and a coupon for a restaurant (apparently the person who lost his wallet worked there, though I have no idea). The ticket validating machine in the tramway doesn't work: do you go tell the driver, or just think "free ride!"?
No, I don't give it back
because I hope I'll get something out of it. If that's my reasoning, I'm probably better off keeping the cash in the wallet. it's still nice to get something besides the feeling of a good act for the day.
It's like DRM: I'm perfectly happy paying for games, but I don't like pirates having a
better experience than me because of crappy DRM - ideally, it'd be the other way 'round.
In-game: in almost any morality-bound game, the "Evil" path will give more return in the short run but closes off some paths later on. In games where being a goody-two-shoes doesn't have
any redeeming qualities, the amount of people playing Light/Good/Tears/Life/... is horribly low.
A way to make Karma visible.
Or, just plain logically: most people are self-interested. Most people lack the fortitude to do good "for the community". Giving people an incentive to do the right thing is useful, as it causes more people to do the right thing. The more people do so, the more chances of it happening to you (e.g. you get your walelt returned with the money in it). The more people see that it can work as long as most/everybody is "good", the less chance of people being selfish and being "bad". Which improves society as a whole.
A hundred years ago, people were "good" because otherwise you'd go to hell. Religion as a basis for morality is severely in decline (or gone almost completely, in Western Europe). It's been proven time and again most people lack the qualities to set up a personal morality system that works without an outside influence. Citizenship and neighbourship are nice, but playing on people's selfishness just
works better.