Plus, the associates program has other benefits for Amazon. As the article points out, authors use affiliate links to promote their books for sale on Amazon so they can get a higher cut of the profits. Seems like that scheme, not even limited to books, would motiviate a lot of people to sell their wares on Amazon.
And then there's the benefit of squeezing out other affiliate link programs. An ad can only have one affiliate link, so by making the Amazon program attractive they out-compete other stores in the ads with affiliate links marketplace.