You might be right, but can you imagine an employee handbook that says "all conversation not related to work is prohibited"? Even air traffic controllers occasionally make quips on the radio.

It can be true for the purposes of harassment that enforcement can be inconsistent, and that can work to a certain degree. When a matter such as this comes before a court the spotty enforcement will be a problem for the employer.

Employers exploit these sorts of things all the time to their benefit, and I am sure Amazon has/will - but when a critical mass is reached they will find that the organizer they fired for talking unions on company time will win. This is part of the reason that it takes an extraordinary effort to unionize.