Unfortunately it's not true. There are a number of cons that apply towards people wanting to do the right thing (and usually mixing a bit of greed in as well), to make the pull twice as hard to resist.
Check out the Glim-dropper scam[1]. I've heard it described in multiple variations (diamond ring at a gas station, wallet on the street, etc). Importantly, the mark is both making money, and providing help to someone who needs it.
In the Glim scam, the target wants to get money he doesn't earn (neither by honest work nor by actually finding the supposed lost item) and profit by concealing information that is not supposed to be concealed (i.e. the contact info of the supposed one-eyed man). If he acted honestly, he'd say "oh, lucky you, you're getting a grand, here's the phone number!" and his loss would be $0.
The Glim-dropper scam involves the mark trying make money by inserting oneself as an intermediary who provides no benefit, concealing his true intentions. Outside of HN, that is considered dishonesty.
I don't think it's quite as simple as that, and that you're ignoring all the possible ways it could play out. I saw it explained and performed once on a program where it was a lost wallet of some sort, and if/when the mark did not offer to insert themselves into the transaction, the person who "found" that wallet would explain that they needed to be somewhere soon, but aren't greedy, and would happily split the reward so they don't miss their meeting.
In this case, the mark is helping not only the person who lost the wallet, but also the finder by allowing them to gain some reward money while not actually having to return the found item.
In this case the mark has real reasons the could expect some financial gain:
1) There is an actual task to be accomplished, they must be intermediary for this item and deliver it to the recipient.
2) The recipient may very well be believed to be unavailable for some period, and the mark may feel that the financial loss constitutes during this period is non-zero.
3) There's risk involved, as they must put out some of teir own money for the transaction to take place, even if there is expected return. There will likely be a risk/reward ratio that makes sense internally.
To me that very clearly makes it possibe that the mark is honest and non-greedy and will get taken by the con.
Indeed, the more honest and generous the mark is, the more money the con makes.
But in this scenario, an honest person would tell the finder about the reward and give him the contact info. It only works if the shopkeeper gets greedy and keeps the reward a secret.
And then the finder miraculously has to be somewhere soon, or is traveling on their way somewhere else, and would happily give up a portion of the reward if they could just be allowed to leave...
At that point, the less greedy the mark, the more they lose.
You're right. The "fake donation" scam is one example. As is the "long lost grandkid, who needs to pay his medical bills". But ... most scams do prey on greed - people assume that it's something illegal, but that it's not themselves getting ripped off by it.
Unfortunately it's not true. There are a number of cons that apply towards people wanting to do the right thing (and usually mixing a bit of greed in as well), to make the pull twice as hard to resist.
Check out the Glim-dropper scam[1]. I've heard it described in multiple variations (diamond ring at a gas station, wallet on the street, etc). Importantly, the mark is both making money, and providing help to someone who needs it.
[1]: http://en.wikipedia.org/wiki/List_of_confidence_tricks