This also enables Amazon to claim you purchased a different item than you did, or charge a different price. The only way to obtain documentation of a purchase is to take a screenshot of every transaction.
I bought some lightbulbs from Amazon recently. I am quite sure I purchased some 7-8 watt LED candle bulbs. I received a package of 50 watt incandescents, which was definitely not what I wanted. I went to my Amazon account and it showed I had purchased the incandescents. I looked at my email receipt, and all contained was is a list of links to Amazon, which led to their site, showing I had purchased the incandescents. The lack of text in the email meant I had no way to determine what I actually purchased and whether the mistake was on my end, Amazon's or the third party vendor.