Even if not a regulation, a formal certification could help. "This is to certify that batch NNNNN contains no bacteria, viruses, heavy metals, organic poisons from the list available at https://...".
As a platform, they can probably get around counterfeit items being listed/sold, but sending one from a different seller seems it might open them up to legal risk if Amazon knows there's counterfeits floating around.
I'm sure we all accept the risk in the fine print somewhere, but that only matters until it makes the news and a politician thinks they can get reelected by going after them
Then you choose which to buy.