As opposed to the industry standard which is linear vesting.
They also have onerous clawback clauses on signing bonuses and relocation, and I know of one instance where the clawback stipulated that the signing bonuses would be paid back pre-tax (e.g., $10k signing bonus, of which you got $6k after tax, but if you quit you have to cough up $10k) - meaning you are actually in the hole if you leave.
They also have a habit of putting 2-year clawbacks on their bonuses rather than the more common 1-year, so leaving even in your second year at Amazon would result in substantial financial loss.
Even in this environment I know multiple people who left before their 1-year anniversary, that's how bad it is in many parts of the company. I personally know multiple people who suffered substantial financial loss in order to leave.