[marcus0x62]

SEO pages tend to be infested with ads (which google makes money on,) whereas legitimate businesses much less so. Imagine this scenario: you run a B&B on a popular tourist island and take reservations over the web. You pay google to run an ad relevant to your business. Some SEO turkey comes along and builds a ranked-up site that shows that ad, and has no other real reason to exist. Does google make more money or less money with the SEO operator in the equation? Would the original business have even bought a google ad if they could get organic traffic to their site without all the SEO spam?

[coliveira]

Your last point is the most important: SEO forces businesses to advertise on Google, because it is so hard to appear on organic results. Without black SEO, there would be much less need for ads. It is clear that Google has a symbiotic relationship with black SEO.