To me that is a real tell. New employees are a risk, but you can tell if they are going to work out after a year. If you are still discounting the RSUs at the second year, it is in hopes that some won't survive to the payout of the third.
This is not me defending Amazon's comp structure (they need to improve it in many ways), just correcting the facts:
At the time of calculating an offer, all 4 years are considered the same "Total Compensation Target" i.e. no built in reduction or raise. Amazon just pays cash equivalents for years 1 and 2 - through a signing bonus to make up for the lack of stocks, and through stocks in years 3 and 4.
The stocks in for each year are calculated assuming an average of 15% growth. This can be problematic, or can be great depending on the year you join.
Yes, technically speaking, the stock only compensation is backloaded. But that is balanced out by a 2 year "signing bonus" that is front loaded an equivalent amount of money, such that you are almost getting a hybrid stock/signing bonus package, so that it is technically the same amount of compensation each year.
You left out a big part here; RSU vesting schedule is back loaded. It's something like 5-20-25-50