I agree with your point, limiting public discourse and transparency through business practices is more malicious than simply trying to increase efficiency by negotiating lower prices.

That said, brave suppliers are capable of shifting the balance of power. Think about it, if "Tamazon" drops an efficient supplier for one of their competitors, Tamazon is going to expose themselves to a competitor who picks up that supplier - likely a competitor who's trying to break Tamazon's monopoly.

Then, suddenly, Tamazon's competitor is offering the consumer a superior purchase option.

If I was the supplier, I wouldn't get afraid, I'd get even. I'd go to every Tamazon competitor I could find and convince them to offer my product at wholesale and then give a scathing testimony to the anti-trust regulator.