Something similar happened to me some time ago. I was displaying ads
on a page, but not earning from it. not eve a penny from that page.
I think you guys are misunderstanding the settings here.
You have two options:
1. "Allow any site to display ads for my account" (The Default)
In this case, any website, even if you don't own it, that displays your ads and gets a click, you would get the earnings and get paid. That, however, also means that you could be penalized if someone decides to use your publisher id on a p0rn site or something. So it's always a risk.
2. "Only allow certain sites to display ads for my account"
In this case, you must specify in the text field all the sites you want to "allow" to display ads using your publisher id. Now, if you don't list a site here, and the site has ads with your id that get clicked, you don't get paid (and according to google the advertiser doesn't pay for the click either). This makes perfect sense, because you would use this setting to protect yourself from someone misusing your publisher id. Now, if someone uses your publisher id on a p0rn site, google will not penalize you and your adsense account will not be at risk since you didn't add that site to your "allowed" list.
So, it's not really an "Adsense Trick" or some sort of Google scam or anything like that. Just a simple setting made for the publisher's convenience.