Whatever this may be, it's not "blackmail", and I would strongly urge you, Nate, to remove that word from the title. Blackmail is defined as "extortion of money or something else of value from a person by the threat of exposing a criminal act or discreditable information. ". Have these authors committed criminal acts which Amazon are threatening to expose?
Saying "we won't sell your books unless you let us print them" certainly is NOT blackmail. It's Amazon's store, and they have a right to dictate whatever conditions (within the law) they wish on authors who want to sell through them. Just as authors have a right to turn round and tell them to go to hell, and choose to sell their books elsewhere.