In my experience when I was younger, low skill labour in warehouses often were employed with a wide net. Hire just about anyone and then fire the ones who show up drunk, high, not at all, slack off, pick fights, etc.
It's hard not to sound politically incorrect but these jobs attract unemployable people and they end up doing unemployable things and find themselves fired. My wife has similar stories about a Chrysler plant but holy cow those ones are unionized and never get fired.
I don't have all the facts but I'm just saying that on the surface, this is just kind of how it works. For all I know there may truly be something rotten and Amazon is being far more of a slave driver than is normal.