Amazon branded products have an advantage through access to analytics, not cost of doing business. Amazon issues internal chargebacks to subsidiaries and business units for use of their platforms.
Still, even with those charges, Amazon can just choose to take losses knowing they aren't really losses. They can run that brand unprofitably and if the losses are coming from distribution fees being charged internally, well, the company still comes out ahead.
And the internal analytics is really a much worse abuse. I suspect that grocery stores make their internal brands the same way, and again, I'm strongly opposed to that being allowed.