Ask HN: How do you justify Google’s SEO policy of not allowing paid links?

You own a website, I own a website. I pay you $100 to show your visitors a link to my website. =BAD You own a website, I own a website, I pay $100 to Google who decides when to show my ad on your website and keep $60 for itself. =GOOD How do you justify Google’s SEO policy of not allowing paid links.