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.
Disclaimer: Worked at Amazon for 9 years.
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