Could it be that this has to do with Obamacare? I read that it is cheaper to have part time employees in the US right now, but I might be wrong on that.
Well if it was Google or Facebook I certainly wouldn't entertain this thought but with Amazon? They are known to do things that some (not me) regard as pretty shady to shave off costs wherever they can.
I wouldn't object to that as Amazon didn't create the rules, they are just operating within the system in the most efficient way they can.
What I find concerning is that the rules are changed in the West to incentivise part time work (which doesn't pay living wages in other sectors than IT) over full time work, effectively leaving more and more workers dependant on welfare.
It's what we see here in the EU too since the crash in 2008. Many jobs were lost and the recovery was mostly in part time jobs where the government has to support these workers with additional payments in order for them to survive.