Yep, that jives with what I've heard from current and past Amazon employees.
Most last 1-2 years, because that's what it takes for the stock options to vest and so you can leave without having to pay back relocation assistance.
As far as corporate culture goes, they do have a problem with excessive middle management, and those are probably the ones that want to keep their employees physically close to them so they can watch them and keep their metrics up. There's a lot of internal competition and finger pointing. Brilliant friend of mine tried putting in for an internal transfer within Amazon, so then his manager put him on a probation plan to prevent losing him to another internal team. Then finally after they had a falling out over this, the manager had him dismissed as a scapegoat for some api outage. A bunch of his other Amazon friends had also gotten scapegoated as well, often for existing problems that they were trying fix, but had been hushed or postponed during planning meetings. Then when it finally dies, the manager has to choose a scapegoat or else lose their own position.
Plus, the only perk Amazon employees seem to have is location. They've built up a pretty nice urban campus in South Lake Union over the past few years, so young'uns can live and work in condos downtown with their little doggies ... and that's about it. They offer few of the competitive perks offered by other Seattle area companies... in the name of frugality employees have to pay for their own coffee and beer in the lounge, and little stuff like that. I guess you get a free Prime membership, but not really any other lucrative "employee discounts" on stuff. So yeah, many, many employees just take the relocation money, hang on for the year or two that it takes to get out, and GTFO.
Like MS, Amazon is probably doing this on purpose to flood the local tech job economy with tech workers, to overall reduce the wages they have to pay the star software devs. The Seattle tech job market is still very tight, but all it would take is Amazon announcing a 5% layoff to completely bomb the market with tech jobseekers... though I'm not sure how much of a spike that would be from their normal attrition rate.