> If I pay for the whole book
In this case you are not paying for the book. As the article states: 'It will affect self-published authors on the Kindle Unlimited and Kindle Owners' Lending Library." So, only books you are lending or renting from Amazon, not those that you buy. Remember, Kindle Unlimited users are free to download as many books (on the list) as they want, for a fixed monthly fee.
> To me, this just looks like another money grab by Amazon on the backs of the authors.
Amazon are not reducing the size of the royalties pie, they are just changing the way it is distributed. Authors who write books that users read will receive more, whilst authors who write books that users don't read will receive less. Personally I can't see this as anything other than a Good Thing.
Of course, if you think Amazon is using some (other) trick of false accounting to screw the authors, then this is quite another (unrelated) matter.