(Added:) Allegations of Google's manipulating the search results to influence voting behavior are much more interesting.
The most remarkable difference was that you can make incremental A/B styles on an Adsense campaign but you can't do that with SEO not least because Google has patented methods to cause your rankings to go haywire whenever you change anything about your site -- one reason why sites like Reddit can go a decade without a major design refresh, if you've got a site which does well you want to keep growing it but whatever you do don't change the link structure.
SEO is a matter of investing in content and link building to get traffic, since Google is in the business of selling traffic, they don't want you to invest in anything other than Adsense. It's like how Facebook doesn't give commercial entities a lot of visibility unless the pay up. I remember when Zynga games got huge with games that would spam you about everything your friends were doing so you'd want to play and back then Zynga was making big $$ and Facebook nothing but https://en.wikipedia.org/wiki/Sheryl_Sandberg but a stop to that.