I think the OP's point is that you have to pass some value to the initial user in order for it to provide value for the person reading the tweet. My only thoughts are the following:

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...