It's hard to say if specifically someone did a cost/benefit/value analysis.

But when you work in a culture of doing things that are frugal, cheap, and those are rewarded, you will be rewarded for doing things that SEEM frugal and cheap.

So even if there is long term value reduction, no one realizes this, or has done the math, and thus it seems like Amazon is saving a butt-load. After all, the face value (in terms of salary) and flexibility of temp workers seems like a win-win, especially emotionally.