That's not quite what I've been reading. I believe the more common claim is that Google has a disincentive to algorithmically weed out the kind of drivel that exists for no other reason than to make its publisher money via AdSense. It's about aggregate effects, not failure to clamp down on individual sites. Or, put another way, it's not if certain sites are serving Google ads, it's because that kind of content is usually associated with AdSense.
AdSense is definitely a problem for search quality. It creates the same imperative for the content farm as Google Search has: get the user to click off the page as soon as possible. And the easiest way to do that is to create high-ranking but unsatisfying content with lots of ad links mixed in.
If Google did not operate AdSense, it seems hard to believe the company would not have penalized this sort of behavior ages ago. A love for AdSense is probably the single largest thing spam sites have in common worldwide.
Disagree. Our quality guidelines at http://www.google.com/support/webmasters/bin/answer.py?hl=en... say "Don't create multiple pages, subdomains, or domains with substantially duplicate content." Duplicate content can be content copied within the site itself or copied from other sites.
Stack Overflow is a bit of a weird case, by the way, because their content license allowed anyone to copy their content. If they didn't have that license, we could consider the clones of SO to be scraper sites that clearly violate our guidelines.