If the website is not part of the DomainPark program, then it is against the adsense Terms of Service to have adsense on a page with no content (like a thank you page, registration page, or other type page with no content).
Sounds like you are referring to a scraper type site - a built for AdSense site.
Most dont last too long in SERPS anyway. I have read other forums that indicate that business generated for the actual advertisers from this type of site is actually suprisingly good (whether this is better than average or not would depend on topic/niche).
Perhaps this may have something to do with it.
Of course, with every click Google makes money - so why limit clicks?
If you see a site that's clearly abusing the system, let Google know about it. The main reason this happens is that thousands upon thousands of sites run AdSense, and if they hired enough staff to police every site closely then it would take a big bite out of their profit margin.