That's been a webmaster question for many years - ever since Google started their war on paid links. Some possible answers:
1. Google is anonymously involved in the paid link market, just to learn what's going on.
2. Google models a "big picture" profile of paid links, seeded by known examples (discovered in step #1) so that they can build an algorithm that spots similar suspected paid links.
#2 would then require human intervention to look more closely at the suspects that were uncovered by the algorithm - and most penalties would be manual actions rather than automated actions. Doesn't that sound a lot like what we are currently seeing?