Amazon typically offers cash bonuses during the first 2 years that bring total comp up to a similar amount in the years where 40% of stock vests. They manage towards a target compensation number. The stock vest schedule just changes the mix of cash vs. stock.

Given they backfill the first two years with grant value bonuses, this is irrelevant: the first two years just pay straight cash to make up those % to 100. First year is an up front payment, second is monthly payments. If anything that vest schedule retains people for those two years rather than being a way to pay less.