I don't mind Amazon knowing everything I read on my Kindle. They use this information to suggest other books I might like, based on those I've already chosen. This benefits me by exposing me to new content I might otherwise have missed. Most importantly, the data is kept between Amazon and me, and it would be contrary to Amazon's best interests as a merchant to sell or expose the data to others.
While Amazon is a merchant, Google is an advertiser. It's in Google's interest to disseminate my personal information to anyone willing to pay for it. In my opinion, there's a huge difference between those business models, and it directly impacts on my privacy. Amazon wants to monetize my personal preferences, but in doing so they're motivated to keep my secrets. Google wants to monetize my data because selling data (and analytics) is what they do - it's how they make money. So Amazon's business model is served by using my data but keeping it private, while Google's is to sell my data to any and all who will pay for it.
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