I think it is mostly about maximizing scale (and lower costs) for the shipping and logistics business. It also used to be that buying from Amazon as the seller was safe but that has gone by the wayside as they decided it was worth the risk to not have to manage separately. I think they will soon have to roll that decision back. At least then their brand stays strong and the get economies of scale for logistics for the third parties.
There was also a story recently about the number of people who post stuff on ebay and just drop ship from Amazon directly. Then the customer gets the product and thinks "this is just from Amazon" and looks up the price. The seller had marked it up and the buy say "bullcrap" and returns the original item and buys it direct from Amazon. However, the actual OEM now has countless returns for no reason. The seller was just running a business on top of Amazon. This is something they have to stop.
There was also a story recently about the number of people who post stuff on ebay and just drop ship from Amazon directly. Then the customer gets the product and thinks "this is just from Amazon" and looks up the price. The seller had marked it up and the buy say "bullcrap" and returns the original item and buys it direct from Amazon. However, the actual OEM now has countless returns for no reason. The seller was just running a business on top of Amazon. This is something they have to stop.