As Graywolf said, Google’s motive for cleaning up the SERPs is self-serving and revenue-driven. Robert Scoble explains in more detail:

Why does Google care? Well, Google’s relevancy rankings will be hurt if people can buy their way onto their pages instead of earn their way to those search results pages by doing the best content, etc. Lots of people are doing comparisons of Google’s search results to Yahoo, Ask, and Microsoft’s search engines. If Google’s result set isn’t the best Google’s market share will start to go down as people figure out there are better engines out there. That, in turn, will hurt Google’s advertising business.
Not to mention that if advertisers know there’s a cheaper way to get onto Google’s search engine than by buying an ad, they’ll go with that system. So, Google has a LOT of incentive to swat down PayPerPost and pay-per-link style systems.

Aaron Wall also points out that Google would make more money on Adwords if Google makes search results harder to game:

The more I think about it the more I realize why Google doesn’t like the various flavors of paid links. It has nothing to do with organic search relevancy.

Not quite. Google cares alot about organic search relevancy. But here’s the catch. As I commented on Aaron’s blog,

Google wants informational sites on the organic results front page while forcing commercial sites to battle it out in the right column. Searchers looking for information on “coffee” will be happy with his organic results (Wikipedia, nationalgeographic, coffeereview, coffeeuniverse), and searchers looking to buy coffee will click on adwords and buy.
If commercial sites show up in organic results, there’s no reason for people to click on Adwords.
That means Google not only wants highly relevant organic results, but highly relevant results that are also non-commercial.

(Time to write meatier, more “informational” articles and dump those aff links from thin pages, huh?)
So Google is smacking down on paid links to increase its profit margin.
So what?
Do you read SEO blogs because they’re fun to read? Do you buy Adword ads because you want to help people find better products? Give me a break. I run ads to make money - pure and simple. And just because I have selfish reasons for giving you a great product doesn’t take away from the value of my product. Sure, Sony makes money off selling PS3s - so what? You want them to make them for free?
We’re all in it for the money, so “Google is selfish” objection doesn’t wash.


Google?s Motives Are Selfish - So Are Yours and Mine - Read More...