Wow, I've always heard terrible things about Amazon burning their employees into the ground, but this sounds like the opposite. Kudos for trying something different.
Maybe I'm too cynical, but the first thing that came to mind for me was: "Just few-enough hours that employees have no benefits."
The magic number for the benefits cut-off is 20 hours/week.
I'm not sure what you're referring to -- Amazon policy? -- but the usual critical-point is 30 hours, for full-vs-part-time legislation and stuff like the ACA.