FYI. I work with an Amazon author and he received an email asking if he wanted to "opt-in" to the program (which he did). So it's completely at the publisher's or author's discretion which I think is the right way of doing it.

Also the author has the ability to set the price for the Kindle version. It's dependent on the retail price but from what I've seen if the Kindle book is >$6 the author can do $2.99, $1.99, $0.99 or free. Then as the price gets lower (>$4) the options narrow to $1.99, $0.99 or free and so on.

It's open to self-published authors. I just opted-in to make by eBook 99 cents for anyone who buys the paperback.