It's not so much that Amazon specifically is the issue, it's the vendors selling through Amazon or the fact some people will buy a real knife and then process a return with a fake or clone knife. Amazon employees are not knife dealers or experts. They likely take a quick peak in the box and say, "yep the knife is in there" and then restock it. I buy stuff from Amazon all the time and returns are a breeze. I just won't buy a knife from them unless they are my only option and I have to have the knife.
It is also Amazon.
You have unscrupulous sellers gaming Amazon's distribution/return system with things like the "same zip-code scam." Not sure if still the case, but Amazon only verifies that an item makes it to your zip-code and is signed off on via USPS/UPS. Seller sends cheap item to wrong address where it is signed as delivered (e.g. you buy a laptop, they send a toaster oven). Wrong address either signs off on the delivery, or there is no signature on delivery and USPS/UPS signs that it was delivered. Then you have to navigate a Kafkaesque customer service labyrinth that denies you, at first, based on their delivery system. You escalate the claim... you then either get lucky and get your money back quickly, get your money back after a very long hassle, or don't get your money back at all, give up, etc.
For items that are utilizing Amazon Prime (third party sellers can use this as well using the Fulfilled by Amazon program), all items are stored in an Amazon distribution center ... together. In addition, Amazon sees all of these items as being a part of the same inventory. So, a reputable third party that is authorized to sell Spyderco, Benchmade, etc. can send in their kosher inventory... but that inventory is then mixed with other sellers inventory which could be anything. Now, you might specifically purchase from the authorized dealer when you conclude your transaction in the cart, but if they allow their goods to mix via the FBA program... you could be getting a knockoff sent in by the seller of ill-repute.
Granted, the above knowledge is based on data from a couple years ago. They might be cracking down on that stuff now... who knows... but I wouldn't bet on it.