On the top you have automation. They have jobs because they cost less than robots. But every wage increase or company benefit or break that reduces worker productivity is a step closer to cost parity with automation, and therefore the unemployment line.
On the other side, they can move away from that line by accepting lower pay or worse working conditions. And there is a trade off between the two as well -- if the job sucks more in a way that allows the business to hire fewer people then they can each get paid more without crossing the line where the jobs get automated.
Blaming Amazon for this is like blaming the local power company for world energy prices. They're just providing one corner of the triangle -- you can work there and have a hard job but better pay than most other unskilled work. If you want an easier unskilled job, those exist too, and they don't pay as much.
But the option to do easy unskilled work for high compensation isn't there. That's the corner of the triangle where the humans cost more than the robots. And maybe that's for the best, but the workers don't need Amazon to actually do that to get the same outcome from their perspective -- they can just quit.