Probationary periods are a thing, and in fact are the default form of hiring in many countries. If the intention of the policy was what you said then Amazon would have told these candidates "we are hiring you for a one year period, and will then evaluate your performance and decide whether to keep you or not". Their comp plan and stock vesting structure would have reflected this as well, and they could then make their decision with all this info.

The fact that "hire to fire" wasn't even an official policy but rather an off the books stunt pulled by managers should tell you everything about it.