How so? I'd be interested to learn more about this. Can you share some sources?
Essentially, Amazon either wants you to A) register a trademark and your brand (the only way to list things without UPC's and use amazon's internal "fxn" label scheme) or purchase UPC's for each product you sell. UPC's are intended to be the same for your product wherever it ends up on the web and are intended to uniquely identify your products. Technically you could purchase a new UPC for each place you list your items, but that can get expensive. Amazon leverages UPC's to identify items in their warehouses and more importantly trace counterfeit items or food items that could require a recall. It used to be common for people to re-sell existing UPC's but recently amazon has started banning sellers for this practice. Having expensive UPC's is also a way to make the barrier to entry higher, making sellers on Amazon really have to want to sell on amazon.
The game theory and psychology of it all is pretty interesting.
Also, UPC codes are entirely controlled by a single company called GS1, start at $25 each. It's also worth noting that GS1 has been compromised multiple times, allowing overseas hackers to list "copycat" products with cloned UPC's defrauding legitimate sellers. Ironically, invalidating the whole reason UPC codes were invented in the first place.