Any employee taking this option is a fool. They would be voluntarily giving up the (sometimes meager) benefits of being defined as a full time employee under US law. Great for Amazon, terrible for the employee.
Under 32 hours and the law would say no benefits are required. Amazon is actually giving them a straight ratio of benefits instead of dropping them to part-time. It's the opposite of a dickish move, as far as the law is concerned (and Amazon is showing that the law need not dictate when businesses are competing for employees).
There are probably many parents who will jump at this kind of opportunity (plus others who want to start a business, do more volunteering, or just have more leisure time).