But that's where customs should come in; counterfeiting products is illegal, selling fake brand clothing is illegal, and if Amazon sells them they can (and should) be held accountable. It's that branch of law enforcement's job to protect both consumers and brands from counterfeits, you pay taxes for that reason.
However, when someone DOES notice, the situation must be remedied. Amazon should be liable to some degree for everything they sell. Consumers should have recourse. Maybe they already do? Can they return for a refund from Amazon if they dispute the authenticity of the product?
Yes, very easily but that's not enough because Amazon still profits so massively from everyone else that it doesn't even raise a bell in their accounting.