So a couple things:
1) Amazon's "stock grants" are RSUs - "Restricted Stock Units" that only become stock after you've vested. Vesting at Amazon takes so long that they've been criticized by Time, CNBC, the New York Times and a couple books for being so stingy. Yeah they give you two RSUs when you hire on... and then an additional RSU at each anniversary. After one year, you're 5% vested. If you could sell 1/20th of an Amazon share, you'd be able to but you can't. After two years you're 15% vested. If you could sell 3/10ths of an Amazon share, you'd be able to but you can't. After two and a half years you're 35% vested - if you could sell 7/10ths of an Amazon share you'd be able to but you can't. After three years you're 55% vested - AND HOLY SHIT YOU HAVE THREE SHARES! You now have one whole Amazon share that you can sell. That's a thousand dollars right now. But up until this point, Amazon has paid you zero in bonuses. To be fully vested you have to stay there four years. Nobody stays at Amazon four years. More than that, Amazon gets to claw back everything they've given you if you quit within 9 months of starting. And all the warehouse workers are contract employees whose hiring term generally runs in weeks, not at-will.
2) Amazon's monthly bonuses are for meeting production quotas intended to make people quit within a few months. They benefit from churn because they use itinerant labor.
But a third thing:
While the minimum wage increase helps out shift workers in the short term, you can bet your sweet bippy that Amazon wouldn't have signed on if they intended for it to matter for very long. Amazon increasing minimum wage for workers is an excellent sign that Amazon intends to invest even more heavily in automation - after all, they bought 75,000 robots last year - and now that will be seen as a cost-saving measure also.
And yeah. Now the Stanford grads making their bones by working for Amazon for a while will have even less to show for it. As I recall something like 5% of Amazon employees ever vest. Now that number will be lower.