This seem to make a lot of sense, so I assume there must be some serious reason for the fact it isn't happening already. Is this hard to implement legally? Is there just not enough incentive to do so?
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.