Perhaps it is important for them to maintain an aura of neutrality and cold calculation. Though if we are supposing that Google is intentionally crippling it's algorithm, would it be so much to suppose that they would just clandestinely run the spam sites themselves to eliminate the middle-man? Though perhaps the cost of secretly doing it themselves is higher than the AdSense payments made to the sites.

This all assumes that Google's reputation remains untarnished by merely linking to spam rather than filling their own site with spam, but I don't think that's quite the case. Google's reputation is intricately tied up with the quality of the results it gives.