I've also wondered if the 5% back isn't something of an offset for the sales tax they've agreed to collect in some states. It doesn't totally cover the difference in high tax states versus ordering from someone else with free shipping with a 1-2% rebate card but it makes the delta small enough that many people may just default to Amazon.

Doesnt apply to me. Been with Amazon since 2005, prime 2 years now, bought a table that turned out to be scratched, open az claim, amazon said work with seller, seller was playing games for 2 months, no refund no return label for havy item that he promised is free to return, I went to bank and disputed the whole thing. Amazon didnt even blink, but it shows u your profile of a perfect amazon client might also ended up in dispute hitting their CB ratio.

Also as of this year Costco lost me as a client since they dont accept Amex anymore. To me that was enough.