Yes, but Kirkland's signature comes from the same factory. If I'm the factory owner and Costco vis going to guarantee me sales albeit at a slightly lower margin, so long as I slap a different sticker on it, that's different than from Amazon finding out which of my products sells best and then gets someone else to rip it off so I don't get paid anything.

First of all, we don't know which factory kirkland's products are coming from. Even if they are coming from the same factory, who guarantees the same ingredients and quality control was used???

everything from amazon is coming from China, I dont understand why does a random person who resells stuff from Chinese factories via Amazon FBA feels entitled for exclusivity arrangement with Amazon?

Was such exclusivity encoded in some form of legally enforceable agreement ?