Looking at lists of "best employers," the one I'm seeing, that comes closest to Amazon in its business model, is Overstock.com. A web site, and low price order fulfillment, seems all that company is, no?
See:
http://www.ksl.com/?nid=148&sid=13791980
The biggest complaint about Overstock, as an employer, seems to be pay. If you treat people well enough, you can pay them somewhat less.
Amazon warehouse workers seem mostly to be employees of Amazon contractors. Since the contractor is going to take a cut, Amazon's actual labor cost are plausibly higher than if people liked working for them.