So if Amazon contracted with same manufacturer and just took some of the margin for distribution, that'd be OK (or at least less bad)?
When I sell to CostCo, I'm very happy to manufacture their house-branded vitamin C in exchange for them carrying my premium name brand vitamin C in stores as well. I make money on both products. (Heck, I as the manufacturer pay money out of my own pocket to advertise that CostCo branded vitamin C.)
Private label manufacturing deals are as secretive as possible so the owner of the market (in this case, Amazon) can change the supplier when it suits them giving them leverage in the negotiation. I'd bet you can find that exact AmazonBasics cable you just bought from the same supplier with a different name on Amazon, too.