Just about every large company has low performing employees, and just about every large company (at least in the U.S.) has a PIP process that attempts to help redeem employees who are identified to be at risk of flunking out. Some individuals, for better or worse, are just not fit to work in a particular role or at a particular company, and fail the PIP. This system is not unique to Amazon, and it's very common in the industry.

About 15 years ago, I was placed on a PIP for low performance at a large technical employer. I managed to climb out of my hole and shine, but it's a good lesson in humility and responding constructively to feedback.