But ... if they apply the same criteria to AdWords landing pages and pages in the SERPs, the only way to get a page into AdWords would be to create a page that would end up at the top of the SERPs anyway? Wouldn't that eventually kill AdWords?
Okay, I'm being facetious, but there's some truth in the above statement.
And the truth is that Google has no real way to judge the quality of the landing page and probably doesn't care. I did some testing recently creating a new page and an ad linking to it. Each keyword was inactive with a ridiculously high minimum bid. No robot visited the landing page. I tried linking the same ad to a different web page - an established page with more on-topic content and the minimum bids were the same. My conclusion is that Google's algorithm simply decided the keywords aren't worth anything to them.