It is fraud in two ways: It misrepresents the seller, and it misrepresent the origin.
That it misrepresents the seller is simple: You don't know who provided you the inventory that Amazon is shipping you. You will often get inventory provided by company B when you receipt says company C. When and if you get a defective product, you have a legal case for compensation against company C, but when they didn't provide the product in the first place (company B did), the waters get muddied. You still have a legal case against company C (They're the seller of record, and you got a faulty product). Company C has a legal case against Amazon (they sent a faulty product on behalf of Company C), and Amazon has a legal case against Company B (They were provided a product that was misrepresented)
There's an alternate line of legal reasoning: Make Amazon the seller of record. This cuts the gordian knot quite nicely, but Amazon has repeatedly rejected it, because:
Sending you commingled product is fraud when the product is not represented correctly: When you buy a "Genuine Apple Charger" and you get a cheap piece of junk in similar packaging, you are being defrauded. Whenever Amazon commingles a non-genuine product with a genuine product and sends it on to the consumer, that's fraud.
This is a legal monty-hall problem. If you lay it all out and examine all of the branches individually, it is quite clear: At some point down each of the lines, Amazon has committed fraud. Either fraud against the consumer, or fraud against it's partners, but regardless: Harm has been done to the consumer and that harm was introduced by Amazon. The harm was introduced when Amazon bucketed the faulty products with the non-faulty products, and thereby destroyed the chain of custody of where it came from. IFF Amazon took careful notes and marked each item with it's origin, then they could pass the blame on to Company B (Where it really belongs), but they didn't, and as such are liable for Company B's misdeeds that they have laundered.