That seems like an overly simplistic anlysis, unless an employee who quits or gets fired not on an anniversary receives a pro-rated partial share at departure.

Otherwise, a separation before the first anniversary (presumably a certainty for seasonal workers) results in losing none of the raise, at just shy of the second anniversary only one quarter of the raise.

Half the raise is an upper bound, and we'd have to know at least the average tenure of a warehouse employee to guess how much an average worker loses.

This doesn't take into account that the stock didn't begin to vest until a worker was at Amazon for one year, and with such high employee turnover few made it to that mark.

tax free? how?

You mean you can pay only cap gains if you hold it versus income tax?