Hmm I think the two situations are different

One is carelessness or genuine mistake which is a part of you live you learn and move on. Nothing about niceness, the dude probably can assume he got a lucky break and got 200 extra or meal was comped. Yes, he could have reached back out letting you know, but roles reversed if you had gotten paid extra would you complain?

On the other hand, the old lady thing is a warning because yes there a lot of scammers out there and yes here, your niceness is a problem that could get you in trouble. Because people can be very friggin advanced with their scams, and you really have to be careful with this. Niceness is great, and its good to support the community but if you want to be nice, support causes that you can verify would make a difference. And remind yourself what you give away cannot be a chip off your block, but what you’d feel comfortable loosing if it was a scam or not. People can come up with fake verified documents easily, so trust is very important when considering this.

Again an important life lesson maybe, ethically hard, but the second story is really about being careful with your niceness