And another questionable thing that is starting to happen more and more is that you get something for free from the seller if you leave a review. It's very difficult for Amazon to guard against this, because the offer is made in the parcel you receive.
This doesn't seem to be that hard to guard against. Amazon could offer a large bounty to report sellers that do this, and since Amazon has direct access to seller funds they could debit their account to pay for the bounty.
Personal opinion: They have consumer confidence to burn, and selling these products is almost pure profit. Amazon doesn't have to source or pay up front for these materials, and only has to pay a minimal storage/shipping fee on them (where they even store the items themselves).
It's hard to turn that kind of profit down, or to slim down the profit by adding more checks and balances.
It's not even hard: just require anyone selling on Amazon to wire a large enough deposit before they are allowed to sell (magnitude depending on the price, volume of their items and possibly their assumed trustworthiness); then, if they are found selling counterfeits or otherwise misrepresenting their products or their company, Amazon pockets a third of it, rewards another third to the person reporting it to Amazon and distributes the remaining third to affected shoppers.
It seems to me that the gains would far outweigh any possible reduction in the number of legitimate sellers and products.