For their core engineering, BD etc... teams in Seattle, retention is great.
Low skill, yes, low pay, God yes, seasonal - not usually. At least where I am, they aggressively hire for full time positions, and clearly intend for non-temporary employees to consider it as a career. Retention is poor because of the environment and the workload.
And theft. Apparently there's a lot of theft. But that's bound to happen when you surround people with expensive merchandise, work them like dogs and refuse to pay them a decent wage.
This is pretty good rant from the technical\engineering side: https://plus.google.com/+RipRowan/posts/eVeouesvaVX
There is no need to fetishize amazon and if amazon is your model I feel sorry for your employees.