Although nobody else seems to have mentioned it, this sounds to me like they are trying to avoid employees being dissatisfied but sticking around in order to get severance pay. Obviously if you quit voluntarily, no severance pay, but Amazon gives you a bit of money anyways (much less than severance pay), so that maybe you won't stick around longer than is good for either of you.
Hmm, I'm in Canada, and while the legal minimum here probably isn't much higher than the US, maybe companies tend to offer more. Could just be what I've heard of in the largish engineering companies I've been in.
In general, there is no requirement for any severance in the U.S. It can be negotiated for, and in certain specific cases laws may require either notice or severance.
