What is the benefit here to the tweeter? I guess you could post an affiliate link and get paid for it. Otherwise you're just advertising for amazon for free, which hardly needs doing.
I personally don't see any need for this in my life right now, though I do buy a lot on Amazon.
For marketers, this could be a huge deal.
Even people like small authors could do great with this: they can have a smooth transition from talking about their book with people online to having them buy it without relying on people's keeping their short term memory together so they remember to pick up the book. Authors sell books on their book tours because people don't have a chance to forget to pick up a copy.
If you find it annoying, you can just either not follow the Amazon accounts or block them. This is a nifty little thing, and it'd be a shame if it gets screwed up.
1. Influential person says "Hey check this book I co-authored".
2. Followers don't think twice about what the book is about or whether it's something they should buy, they'll just insta-add it to their cart. This is the most frictionless and convenient way to do so.
3. Amazon probably sees huge value in getting items into consumer's carts, since it means they can optimize for conversion. oh you're interested in <Famous person>'s newest book? you might like XYZ as well...