The cost of suing Amazon (which is how a customer would hold them liable) greatly outweighs the cost of almost any counterfeit good they might happen to ship.

The government is not likely to step with criminal enforcement unless they are presented with proof that Amazon purposefully ordered the fakes, or knowingly sold fakes.

Generally speaking, retailers are considered victims, not perpetrators, of counterfeiting. That's because when a retailer sells a counterfeit, the retailer has to process the returns and deal with angry customers, and then they're left with worthless inventory.

Again, the exception is if the retailer knowingly traffics in fakes and tries to defraud their customers. Proving that takes some pretty specific evidence though.