I'm not a sociologist, but I've had a longstanding interest in the origin of manners, so I've read a bit on this. The problem here is that putting a number on something invites comparisons. We almost can't help comparing numbers. If my friend gets a number and I get a "You are awesome! Thank you so much!" I can't easily compare those because they are unlike each other. But when my friend gets a 500 and I get a 12.5, it is obvious to everyone that the number I got is so much lower, it's embarrassing. That guy's number is way bigger than mine.

Basically, no monetary reward is necessarily expected here. But once you give one, it is a faux pas to give one that looks small in comparison to what is considered the "norm." We even have common insults reflecting this fact, where bragging about something I impressive will invite comments like, "Wow, here is a shiny nickel." The impromptu offer of a clearly paltry reward is understood as a slight.

Yahoo "owed him one" when he reported the vulerabilty.

When they traded a shirt for the report Yahoo was implying that they no longer owed him anything, it was a fair barter

Yeah, I'd like a better explanation myself. I feel it, but I can't really explain why.

It seems like there are just two distinct modes. When you do something for free, you're basically doing a favor, and you get pleasure from being altruistic. Once you get paid, it's now an exchange of value, not altruism, so you only get pleasure from being paid. If the pay is small, you'll feel upset because your end of the bargain is based entirely on that, while if you're doing it for free, you switch over.

I could be full of it....