I never said "not the people committing the fraud". Why not both? Amazon is the entity warehousing and shipping the products in the United States (or wherever the buyer happens to be), and is and is the party enabling the fraudulent products to be sold to the buyer. I'm not a lawyer, but I'd have to think there are consumer protection laws being violated here, as well as mail fraud, assuming at least some of these products are shipped via the postal service.

I support legal action against the companies selling the products too, but they tend to disappear and reappear under different names, and are likely all located in China, which I assume makes prosecution more difficult.

Responding to your edit: this is not "private companies doing the job of the government." If there are fraud and/or consumer protection laws on the books, then companies need to follow them, and they need to face legal consequences when they don't.

The real issue is that Amazon is not doing anything about their fake review problem for products that they list on their website. Whether or not that has any legal consequence is up for debate.